Setting GRAPH_FACTS causes a 500 error
**Describe the bug**

We set `GRAPH_FACTS = "architecture,puppetversion,osfamily"` in the config file. This seems to cause an error when loading node facts:

```
ERROR:puppetboard.core:Exception on /node/dc4.example.com/facts/json [GET]
Traceback (most recent call last):
  File "/srv/puppetboard/virtenv-puppetboard/lib64/python3.9/site-packages/flask/app.py", line 2528, in wsgi_app
    response = self.full_dispatch_request()
  File "/srv/puppetboard/virtenv-puppetboard/lib64/python3.9/site-packages/flask/app.py", line 1825, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/srv/puppetboard/virtenv-puppetboard/lib64/python3.9/site-packages/flask/app.py", line 1823, in full_dispatch_request
    rv = self.dispatch_request()
  File "/srv/puppetboard/virtenv-puppetboard/lib64/python3.9/site-packages/flask/app.py", line 1799, in dispatch_request
    return self.ensure_sync(self.view_functions[rule.endpoint])(**view_args)
  File "/srv/puppetboard/virtenv-puppetboard/lib64/python3.9/site-packages/puppetboard/views/facts.py", line 50, in fact_ajax
    if fact in app.config['GRAPH_FACTS'] and value is None and node is None:
TypeError: 'in <string>' requires string as left operand, not NoneType
```

**Puppetboard version**

4.3.0

**Environment and installation method**

* Rocky Linux 9.2 with Python 3.9.16 from the OS repositories
* installed with the Forge module v9.0.0 and using the PyPI package (default).

diff --git a/puppetboard/views/facts.py b/puppetboard/views/facts.py
index 17525ca..6649071 100644
--- a/puppetboard/views/facts.py
+++ b/puppetboard/views/facts.py
@@ -47,7 +47,7 @@ def fact_ajax(env, node, fact, value):
     check_env(env, envs)
 
     render_graph = False
-    if fact in app.config['GRAPH_FACTS'] and value is None and node is None:
+    if fact is not None and fact in app.config['GRAPH_FACTS'] and value is None and node is None:
         render_graph = True
 
     query = AndOperator()
['puppetboard/views/facts.py']