chore: Do not treat FinishReason.STOP as error case for LLM responses containing candidates with empty contents

Fixes #2905

PiperOrigin-RevId: 825263302

src/google/adk/models/llm_response.py

@@ -148,7 +148,9 @@ class LlmResponse(BaseModel):
     usage_metadata = generate_content_response.usage_metadata
     if generate_content_response.candidates:
       candidate = generate_content_response.candidates[0]
-      if candidate.content and candidate.content.parts:
+      if (
+          candidate.content and candidate.content.parts
+      ) or candidate.finish_reason == types.FinishReason.STOP:
         return LlmResponse(
             content=candidate.content,
             grounding_metadata=candidate.grounding_metadata,

tests/unittests/models/test_llm_response.py

@@ -317,3 +317,20 @@ def test_llm_response_create_error_case_with_citation_metadata():
   assert (
       response.error_message == 'Response blocked due to recitation triggered'
   )
+
+
+def test_llm_response_create_empty_content_with_stop_reason():
+  """Test LlmResponse.create() with empty content and stop finish reason."""
+  generate_content_response = types.GenerateContentResponse(
+      candidates=[
+          types.Candidate(
+              content=types.Content(parts=[]),
+              finish_reason=types.FinishReason.STOP,
+          )
+      ]
+  )
+
+  response = LlmResponse.create(generate_content_response)
+
+  assert response.error_code is None
+  assert response.content is not None

tests/unittests/utils/test_streaming_utils.py

@@ -179,3 +179,24 @@ class TestStreamingResponseAggregator:
     assert closed_response.content.parts[0].text == "Error"
     assert closed_response.error_code == types.FinishReason.RECITATION
     assert closed_response.error_message == "Recitation error"
+
+  @pytest.mark.asyncio
+  async def test_process_response_with_none_content(self):
+    """Test that StreamingResponseAggregator handles content=None."""
+    aggregator = streaming_utils.StreamingResponseAggregator()
+    response = types.GenerateContentResponse(
+        candidates=[
+            types.Candidate(
+                content=types.Content(parts=[]),
+                finish_reason=types.FinishReason.STOP,
+            )
+        ]
+    )
+    results = []
+    async for r in aggregator.process_response(response):
+      results.append(r)
+    assert len(results) == 1
+    assert results[0].content is not None
+
+    closed_response = aggregator.close()
+    assert closed_response is None