Fix TypeError when cls.__module__ is None

Trying to use `pdoc -o` instead of `pdoc` in my project caused a long
stacktrace with this error at the end:

    File "/Users/feuh/.../.venv/lib/python3.12/site-packages/pdoc/doc.py", line 640, in _var_annotations
        _safe_getattr(cls, "__module__", "") + "." + cls.__qualname__
        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~
    TypeError: unsupported operand type(s) for +: 'NoneType' and 'str'

This was caused by one class' `__module__` attribute being `None`.

I could not find the reason why that happens and why it happens only
when using `-o`, but this change ensures it won't happen again.

Author: iFreilicht

pdoc/doc.py

@@ -1307,7 +1307,8 @@ def _children(doc: Namespace) -> str:
 def _safe_getattr(obj, attr, default):
     """Like `getattr()`, but never raises."""
     try:
-        return getattr(obj, attr, default)
+        result = getattr(obj, attr, default)
+        return result if result is not None else default
     except Exception as e:
         warnings.warn(
             f"getattr({obj!r}, {attr!r}, {default!r}) raised an exception: {e!r}"