Export failure when enabling "Always Sample Animations"
**Describe the bug**
I get the following stack trace when trying to export as gltf.
```
Traceback (most recent call last):
  File "C:\\Program Files\\Blender Foundation\\Blender\\2.80\\scripts\\addons\\io_scene_gltf2\\__init__.py", line 420, in execute
    return gltf2_blender_export.save(context, export_settings)
  File "C:\\Program Files\\Blender Foundation\\Blender\\2.80\\scripts\\addons\\io_scene_gltf2\\blender\\exp\\gltf2_blender_export.py", line 40, in save
    json, buffer = __export(export_settings)
  File "C:\\Program Files\\Blender Foundation\\Blender\\2.80\\scripts\\addons\\io_scene_gltf2\\blender\\exp\\gltf2_blender_export.py", line 54, in __export
    __gather_gltf(exporter, export_settings)
  File "C:\\Program Files\\Blender Foundation\\Blender\\2.80\\scripts\\addons\\io_scene_gltf2\\blender\\exp\\gltf2_blender_export.py", line 69, in __gather_gltf
    active_scene_idx, scenes, animations = gltf2_blender_gather.gather_gltf2(export_settings)
  File "C:\\Program Files\\Blender Foundation\\Blender\\2.80\\scripts\\addons\\io_scene_gltf2\\blender\\exp\\gltf2_blender_gather.py", line 38, in gather_gltf2
    animations += __gather_animations(blender_scene, export_settings)
  File "C:\\Program Files\\Blender Foundation\\Blender\\2.80\\scripts\\addons\\io_scene_gltf2\\blender\\exp\\gltf2_blender_gather.py", line 70, in __gather_animations
    animations_, merged_tracks = gltf2_blender_gather_animations.gather_animations(blender_object, merged_tracks, len(animations), export_settings)
  File "C:\\Program Files\\Blender Foundation\\Blender\\2.80\\scripts\\addons\\io_scene_gltf2\\blender\\exp\\gltf2_blender_gather_animations.py", line 64, in gather_animations
    animation = __gather_animation(blender_action, blender_object, export_settings)
  File "C:\\Program Files\\Blender Foundation\\Blender\\2.80\\scripts\\addons\\io_scene_gltf2\\blender\\exp\\gltf2_blender_gather_animations.py", line 94, in __gather_animation
    channels=__gather_channels(blender_action, blender_object, export_settings),
  File "C:\\Program Files\\Blender Foundation\\Blender\\2.80\\scripts\\addons\\io_scene_gltf2\\blender\\exp\\gltf2_blender_gather_animations.py", line 128, in __gather_channels
    blender_action, blender_object, export_settings)
  File "C:\\Program Files\\Blender Foundation\\Blender\\2.80\\scripts\\addons\\io_scene_gltf2\\blender\\exp\\gltf2_blender_gather_cache.py", line 65, in wrapper_cached
    result = func(*args)
  File "C:\\Program Files\\Blender Foundation\\Blender\\2.80\\scripts\\addons\\io_scene_gltf2\\blender\\exp\\gltf2_blender_gather_animation_channels.py", line 66, in gather_animation_channels
    blender_action.name)
  File "C:\\Program Files\\Blender Foundation\\Blender\\2.80\\scripts\\addons\\io_scene_gltf2\\blender\\exp\\gltf2_blender_gather_animation_channels.py", line 115, in __gather_animation_channel
    sampler=__gather_sampler(channels, blender_object, export_settings, bake_bone, bake_channel, bake_range_start, bake_range_end, action_name),
  File "C:\\Program Files\\Blender Foundation\\Blender\\2.80\\scripts\\addons\\io_scene_gltf2\\blender\\exp\\gltf2_blender_gather_animation_channels.py", line 160, in __gather_sampler
    export_settings
  File "C:\\Program Files\\Blender Foundation\\Blender\\2.80\\scripts\\addons\\io_scene_gltf2\\blender\\exp\\gltf2_blender_gather_cache.py", line 65, in wrapper_cached
    result = func(*args)
  File "C:\\Program Files\\Blender Foundation\\Blender\\2.80\\scripts\\addons\\io_scene_gltf2\\blender\\exp\\gltf2_blender_gather_animation_samplers.py", line 62, in gather_animation_sampler
    bake_bone, bake_channel, bake_range_start, bake_range_end, action_name, export_settings),
  File "C:\\Program Files\\Blender Foundation\\Blender\\2.80\\scripts\\addons\\io_scene_gltf2\\blender\\exp\\gltf2_blender_gather_cache.py", line 65, in wrapper_cached
    result = func(*args)
  File "C:\\Program Files\\Blender Foundation\\Blender\\2.80\\scripts\\addons\\io_scene_gltf2\\blender\\exp\\gltf2_blender_gather_animation_samplers.py", line 184, in __gather_input
    export_settings)
  File "C:\\Program Files\\Blender Foundation\\Blender\\2.80\\scripts\\addons\\io_scene_gltf2\\blender\\exp\\gltf2_blender_gather_cache.py", line 65, in wrapper_cached
    result = func(*args)
  File "C:\\Program Files\\Blender Foundation\\Blender\\2.80\\scripts\\addons\\io_scene_gltf2\\blender\\exp\\gltf2_blender_gather_animation_sampler_keyframes.py", line 195, in gather_keyframes
    while frame <= end_frame:
TypeError: '<=' not supported between instances of 'NoneType' and 'NoneType'
```


**To Reproduce**
- Open the attached blender file.
- Export gltf, with "Always Sample Animations" enabled

**.blend file/ .gltf**
[anim1.zip](https://github.com/KhronosGroup/glTF-Blender-IO/files/3638702/anim1.zip)

**Version**
- OS: Windows
- Blender Version 2.80
- Happens with both version shipped with 2.80 and glTF-Blender-IO git master

**Additional context**
Just for information, this is originally a blender file I got from some website, with an already skinned model. The animation is from me though.

diff --git a/addons/io_scene_gltf2/blender/exp/gltf2_blender_gather_animation_channels.py b/addons/io_scene_gltf2/blender/exp/gltf2_blender_gather_animation_channels.py
index 22b8be68..2ec3e233 100644
--- a/addons/io_scene_gltf2/blender/exp/gltf2_blender_gather_animation_channels.py
+++ b/addons/io_scene_gltf2/blender/exp/gltf2_blender_gather_animation_channels.py
@@ -52,6 +52,10 @@ def gather_animation_channels(blender_action: bpy.types.Action,
     if blender_object.type == "ARMATURE" and export_settings['gltf_force_sampling'] is True:
         # We have to store sampled animation data for every deformation bones
 
+        # Check that there are some anim in this action
+        if bake_range_start is None:
+            return []
+            
         # Then bake all bones
         for bone in blender_object.data.bones:
             for p in ["location", "rotation_quaternion", "scale"]:
['addons/io_scene_gltf2/blender/exp/gltf2_blender_gather_animation_channels.py']