Only create subheading if we have class docs (#4126)

The current docs crash for example when accessing `/documentation/user`.
This PR fixes the issue introduced by #4115.

Author: rodja

website/documentation/reference.py

@@ -10,7 +10,7 @@
 def generate_class_doc(class_obj: type, part_title: str) -> None:
     """Generate documentation for a class including all its methods and properties."""
     doc = class_obj.__doc__ or class_obj.__init__.__doc__
-    if ':param' in doc:
+    if doc and ':param' in doc:
         subheading('Initializer', anchor_name=create_anchor_name(part_title.replace('Reference', 'Initializer')))
         description = remove_indentation(doc.split('\n', 1)[-1])
         lines = [line.replace(':param ', ':') for line in description.splitlines() if ':param' in line]