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.

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