fix: Do not call _append_gapic_content() on empty content

Author: yifanmai

vertexai/generative_models/_generative_models.py

@@ -1829,8 +1829,8 @@ def _append_gapic_candidate(
  raise ValueError(
  f"Incorrect candidate indexes: {base_candidate.index} != {new_candidate.index}"
  )
-
- _append_gapic_content(base_candidate.content, new_candidate.content)
+ if new_candidate.content:
+  _append_gapic_content(base_candidate.content, new_candidate.content)
 
  # For these attributes, the last value wins
  if new_candidate.finish_reason: