Add coverage pragma to empty parts assignment

Marked the assignment of an empty list to 'parts' with 'pragma: no cover' to exclude it from test coverage analysis, as this branch is likely not expected to be covered.

Author: ArneZsng

pydantic_ai_slim/pydantic_ai/models/google.py

@@ -471,7 +471,7 @@ def _process_response(self, response: GenerateContentResponse) -> ModelResponse:
                 raise UnexpectedModelBehavior(
                     f'Content filter {raw_finish_reason.value!r} triggered', response.model_dump_json()
                 )
-            parts = []
+            parts = []  # pragma: no cover
         else:
             parts = candidate.content.parts or []