test: handle clang diagnostics returning None for filename

The parser may return None for clang diagnostics filename if there isn't
one. Handle it gracefully.

test/test_parser.py

@@ -81,7 +81,8 @@ def get_output(self):
             roots[key] = root
 
             for error in errors:
-                errors_str += f'{error.level.name}: {os.path.basename(error.filename)}:{error.line}: {error.message}\n'  # noqa: E501
+                filename = os.path.basename(error.filename) if error.filename else ''
+                errors_str += f'{error.level.name}: {filename}:{error.line}: {error.message}\n'  # noqa: E501
 
         for directive in self.directives:
             filename = directive.get_input_filename()