Fixup "REF: Use pdoc.Doc.source where available"

This fixups commit e9fd89f.

Author: kernc

pdoc/__init__.py

@@ -253,7 +253,10 @@ def _pep224_docstrings(doc_obj: Union['Module', 'Class'], *,
         tree = _init_tree
     else:
         try:
-            tree = ast.parse(doc_obj.source or None)  # type: ignore
+            # Maybe raise exceptions with appropriate message
+            # before using cleaned doc_obj.source
+            _ = inspect.findsource(doc_obj.obj)
+            tree = ast.parse(doc_obj.source)  # type: ignore
         except (OSError, TypeError, SyntaxError) as exc:
             warn("Couldn't read PEP-224 variable docstrings from {!r}: {}".format(doc_obj, exc))
             return {}, {}