fix(vertex_httpx.py): Fix tool calling with empty param list

Fixes https://github.com/BerriAI/litellm/issues/5055

litellm/tests/test_function_calling.py

@@ -337,3 +337,27 @@ def test_groq_parallel_function_call():
                 print("second response\n", second_response)
     except Exception as e:
         pytest.fail(f"Error occurred: {e}")
+
+
+@pytest.mark.parametrize("model", ["gemini/gemini-1.5-pro"])
+def test_simple_function_call_function_param(model):
+    try:
+        litellm.set_verbose = True
+        messages = [{"role": "user", "content": "What is the weather like in Boston?"}]
+        response = completion(
+            model=model,
+            messages=messages,
+            tools=[
+                {
+                    "type": "function",
+                    "function": {
+                        "name": "plot",
+                        "description": "Generate plots",
+                    },
+                }
+            ],
+            tool_choice="auto",
+        )
+        print(f"response: {response}")
+    except Exception as e:
+        raise e