Fix: Ensure that tool calls with no arguments get handled correctly #3560

When a model decides to use an MCP tool call that requires no arguments, it sets the arguments field to None. This causes validation errors because this field gets removed when being parsed by an openai compatible inference provider like vLLM
This PR ensures that, as soon as the tool call args are accumulated while streaming, we check to ensure no tool call function arguments are set to None - if they are we replace them with "{}"

Closes #3456

Added new unit test to verify that any tool calls with function arguments set to None get handled correctly

Signed-off-by: Jaideep Rao <jrao@redhat.com>

tests/integration/agents/test_openai_responses.py

@@ -264,3 +264,36 @@ def test_function_call_output_response(openai_client, client_with_models, text_m
     assert (
         "sunny" in response2.output[0].content[0].text.lower() or "warm" in response2.output[0].content[0].text.lower()
     )
+
+
+def test_function_call_output_response_with_none_arguments(openai_client, client_with_models, text_model_id):
+    """Test handling of function call outputs in responses when function does not accept arguments."""
+    if isinstance(client_with_models, LlamaStackAsLibraryClient):
+        pytest.skip("OpenAI responses are not supported when testing with library client yet.")
+
+    client = openai_client
+
+    # First create a response that triggers a function call
+    response = client.responses.create(
+        model=text_model_id,
+        input=[
+            {
+                "role": "user",
+                "content": "what's the current time? You MUST call the `get_current_time` function to find out.",
+            }
+        ],
+        tools=[
+            {
+                "type": "function",
+                "name": "get_current_time",
+                "description": "Get the current time",
+                "parameters": {},
+            }
+        ],
+        stream=False,
+    )
+
+    # Verify we got a function call
+    assert response.output[0].type == "function_call"
+    assert response.output[0].arguments == "{}"
+    _ = response.output[0].call_id