Fix AttributeError when using Nuitka-compiled models

When using tortoise-orm with Nuitka-compiled Python code, the _get_comments
function fails with an AttributeError because inspect.getsourcefile() returns
None for compiled modules, and the code attempts to call .endswith() on None.

This error occurs in the following stack trace:
- _get_comments() calls inspect.getsource(cls)
- inspect.getsource() -> inspect.getsourcelines() -> inspect.findsource()
- inspect.findsourcefile() returns None for Nuitka-compiled modules
- inspect.findsource() tries to call filename.endswith(), causing AttributeError

The fix adds AttributeError to the exception handling in _get_comments(),
allowing the function to gracefully return an empty dict when source code
cannot be retrieved from compiled modules.

Fixes: AttributeError: 'NoneType' object has no attribute 'endswith'

Author: shkey

tortoise/models.py

@@ -151,7 +151,7 @@ def _get_comments(cls: type[Model]) -> dict[str, str]:
     """
     try:
         source = inspect.getsource(cls)
-    except (TypeError, OSError):  # pragma: nocoverage
+    except (AttributeError, TypeError, OSError):  # pragma: nocoverage
         return {}
     comments = {}