fix: restore tool_calls finish_reason for empty-text final chunk (issue #22900)

The guard condition `choice.delta.content is None` excluded the case where
Gemini sends `parts: [{text: ""}]` (empty string, not None) as the final
chunk after tool calls. Widen to `not choice.delta.content` to cover both
None and empty-string. Also restores the test for this case that was dropped
in the previous commit.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

litellm/llms/vertex_ai/gemini/vertex_and_google_ai_studio_gemini.py

@@ -3120,7 +3120,7 @@ class ModelResponseIterator:
                                 choice.delta is None
                                 or (
                                     not choice.delta.tool_calls
-                                    and choice.delta.content is None
+                                    and not choice.delta.content
                                 )
                             )
                         ):

tests/test_litellm/llms/vertex_ai/gemini/test_gemini_streaming_tool_call_finish_reason.py

@@ -337,3 +337,69 @@ def test_streaming_tool_calls_then_empty_content_finish_reason_is_tool_calls():
     assert len(response2.choices) == 1
     assert response2.choices[0].finish_reason == "tool_calls"
     assert response2.choices[0].delta.content is None
+
+
+def test_streaming_tool_calls_then_empty_text_finish_reason_is_tool_calls():
+    """
+    When Gemini streams tool calls in one chunk and the final chunk has BOTH
+    empty content (parts: [{text: ""}]) AND finishReason="STOP", the
+    finish_reason must still be "tool_calls".
+
+    This covers models like gemini-3.1-flash-lite-preview that send the
+    final chunk with content (empty text) instead of omitting it entirely.
+
+    Ref: https://github.com/BerriAI/litellm/issues/22900
+    """
+    logging_obj = _make_logging_obj()
+    iterator = ModelResponseIterator(
+        streaming_response=iter([]),
+        sync_stream=True,
+        logging_obj=logging_obj,
+    )
+
+    # Chunk 1: tool call with no finishReason
+    chunk_with_tool_calls = {
+        "candidates": [
+            {
+                "content": {
+                    "parts": [
+                        {
+                            "functionCall": {
+                                "name": "get_current_weather",
+                                "args": {"location": "Boston, MA"},
+                            }
+                        }
+                    ],
+                    "role": "model",
+                },
+                "index": 0,
+            }
+        ],
+    }
+
+    # Chunk 2: finishReason="STOP" with empty text content (not None, but "")
+    chunk_empty_text_with_finish = {
+        "candidates": [
+            {
+                "content": {
+                    "parts": [{"text": ""}],
+                    "role": "model",
+                },
+                "finishReason": "STOP",
+                "index": 0,
+            }
+        ],
+    }
+
+    # Process chunk 1: tool calls
+    response1 = iterator.chunk_parser(chunk_with_tool_calls)
+    assert response1 is not None
+    assert len(response1.choices) == 1
+    assert response1.choices[0].delta.tool_calls is not None
+    assert iterator.has_seen_tool_calls is True
+
+    # Process chunk 2: empty text content with finishReason
+    response2 = iterator.chunk_parser(chunk_empty_text_with_finish)
+    assert response2 is not None
+    assert len(response2.choices) == 1
+    assert response2.choices[0].finish_reason == "tool_calls"