fix: handle Gemini responses without candidates

.sampo/changesets/doughty-warden-otso.md

@@ -0,0 +1,5 @@
+---
+pypi/posthog: patch
+---
+
+Fix Gemini web search extraction when response candidates are null.

posthog/ai/gemini/gemini_converter.py

@@ -438,7 +438,7 @@ def extract_gemini_web_search_count(response: Any) -> int:
     """
 
     # Check for grounding_metadata in candidates
-    if hasattr(response, "candidates"):
+    if hasattr(response, "candidates") and response.candidates:
         for candidate in response.candidates:
             if (
                 hasattr(candidate, "grounding_metadata")

posthog/test/ai/gemini/test_gemini.py

@@ -1170,6 +1170,48 @@ def test_empty_array_grounding_metadata_no_web_search(
     assert props["$ai_output_tokens"] == 12
 
 
+@pytest.mark.parametrize("candidates_value", [None, []])
+def test_falsy_candidates_no_web_search(
+    mock_client, mock_google_genai_client, candidates_value
+):
+    """Test that response with falsy candidates does not crash web search extraction."""
+
+    mock_response = MagicMock()
+
+    # Mock usage metadata
+    mock_usage = MagicMock()
+    mock_usage.prompt_token_count = 5
+    mock_usage.candidates_token_count = 8
+    mock_usage.cached_content_token_count = 0
+    mock_usage.thoughts_token_count = 0
+    mock_response.usage_metadata = mock_usage
+
+    # candidates attribute exists but is falsy
+    mock_response.candidates = candidates_value
+    mock_response.text = "Hello!"
+
+    mock_google_genai_client.models.generate_content.return_value = mock_response
+
+    client = Client(api_key="test-key", posthog_client=mock_client)
+
+    response = client.models.generate_content(
+        model="gemini-2.5-flash",
+        contents="Hi",
+        posthog_distinct_id="test-id",
+    )
+
+    assert response == mock_response
+    assert mock_client.capture.call_count == 1
+
+    call_args = mock_client.capture.call_args[1]
+    props = call_args["properties"]
+
+    # Should not crash and web search count should not be present
+    assert "$ai_web_search_count" not in props
+    assert props["$ai_input_tokens"] == 5
+    assert props["$ai_output_tokens"] == 8
+
+
 @pytest.fixture
 def mock_embed_content_response():
     mock_response = MagicMock()