Fix error when an addon has no __init__.py

When an addon has been removed, but its `.pyc` files are still there,
the Python module can still be loaded. However, because `__init__.py` is
missing, it becomes a namespace instead of a module, and its `__file__`
will be set to `None`. As a result, it's impossible to get the mtime
from the file (because there is none).

This should not influence any regularly uninstalled add-on, as that
would just remove the add-on's directory; I ran into the problem when
switching Git branches caused an add-on's Python files to disappear
while keeping the `__pycache__` directory around.

release/scripts/modules/addon_utils.py

@@ -349,6 +349,10 @@ def enable(module_name, *, default_set=False, persistent=False, handle_error=Non
         # 1) try import
         try:
             mod = __import__(module_name)
+            if mod.__file__ is None:
+                # This can happen when the addon has been removed but there are
+                # residual `.pyc` files left behind.
+                raise ImportError(name=module_name)
             mod.__time__ = os.path.getmtime(mod.__file__)
             mod.__addon_enabled__ = False
         except Exception as ex: