composite() error on ex-vector layer
**Description:**

It seems like psd_tools expects a layer with a `stroke` to always have a `vector_mask`.
Photoshop 21.2.6 gives me a psd (attached) with only a stroke (it was a vector layer converted to pixels).
Because of that, I have the following error when I try to `composite()` the layer:
```
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "\\Python\\Python38\\lib\\site-packages\\psd_tools\\api\\layers.py", line 418, in composite
    return composite_pil(self, color, alpha, viewport, layer_filter, force)
  File "\\Python\\Python38\\lib\\site-packages\\psd_tools\\composite\\__init__.py", line 32, in composite_pil
    color, _, alpha = composite(
  File "\\Python\\Python38\\lib\\site-packages\\psd_tools\\composite\\__init__.py", line 102, in composite
    compositor.apply(layer)
  File "\\Python\\Python38\\lib\\site-packages\\psd_tools\\composite\\__init__.py", line 196, in apply
    color, shape, alpha = self._get_object(layer)
  File "\\Python\\Python38\\lib\\site-packages\\psd_tools\\composite\\__init__.py", line 338, in _get_object
    color_s, shape_s, alpha_s = self._get_stroke(layer)
  File "\\Python\\Python38\\lib\\site-packages\\psd_tools\\composite\\__init__.py", line 418, in _get_stroke
    shape = draw_stroke(layer)
  File "\\Python\\Python38\\lib\\site-packages\\psd_tools\\composite\\vector.py", line 42, in draw_stroke
    return _draw_path(
  File "\\Python\\Python38\\lib\\site-packages\\psd_tools\\composite\\vector.py", line 64, in _draw_path
    for subpath in layer.vector_mask.paths:
AttributeError: 'NoneType' object has no attribute 'paths'
```

**To Reproduce:**
```python
from psd_tools import PSDImage
PSDImage.open('Forme_783.psd')[0].composite()
```

**File and screenshots**
[Forme_783.zip](https://github.com/psd-tools/psd-tools/files/8341524/Forme_783.zip)


**Environment**
 - Python 3.7 and 3.8
 - psd_tools 1.9.18

diff --git a/src/psd_tools/composite/__init__.py b/src/psd_tools/composite/__init__.py
index b536f5d..f200aef 100644
--- a/src/psd_tools/composite/__init__.py
+++ b/src/psd_tools/composite/__init__.py
@@ -334,7 +334,7 @@ class Compositor(object):
             color = self._apply_clip_layers(layer, color, alpha)
 
         # Apply stroke if any.
-        if layer.has_stroke() and layer.stroke.enabled:
+        if layer.has_vector_mask() and layer.has_stroke() and layer.stroke.enabled:
             color_s, shape_s, alpha_s = self._get_stroke(layer)
             compositor = Compositor(self._viewport, color, alpha)
             compositor._apply_source(
['src/psd_tools/composite/__init__.py']