Python: FoundryEvals always emits arguments field for tool calls (#7734)

* Python: always emit arguments field for tool calls in AgentEvalConverter

FoundryEvals uploaded tool_call content items without an arguments
field when a tool call had no model-supplied arguments. Foundry's
tool-aware evaluators (task_adherence, tool_output_utilization,
tool_call_accuracy) require the arguments field to always be present,
so zero-argument tool calls caused evaluation to fail with
FAILED_EXECUTION. Default to an empty object instead of omitting the
field.

* Python: only default arguments to {} when None, not on falsy values

Addresses Copilot review feedback: a truthiness check would also
overwrite valid but falsy parsed arguments (e.g. 0, "", False) with
{}. Use an explicit None check so only missing arguments are defaulted.

python/packages/core/agent_framework/_evaluation.py

@@ -797,8 +797,7 @@ class AgentEvalConverter:
                     "tool_call_id": c.call_id or "",
                     "name": c.name or "",
                 }
-                if args:
-                    tc["arguments"] = args
+                tc["arguments"] = args if args is not None else {}
                 content_items.append(tc)
             elif c.type == "function_result":
                 result_val = c.result

python/packages/foundry/tests/test_foundry_evals.py

@@ -153,6 +153,23 @@ class TestConvertMessage:
         assert tc["name"] == "get_weather"
         assert tc["arguments"] == {"location": "Seattle"}
 
+    def test_assistant_with_zero_argument_tool_call(self) -> None:
+        msg = Message(
+            "assistant",
+            [
+                Content.from_function_call(
+                    call_id="call_3",
+                    name="get_site_summary",
+                    arguments=None,
+                ),
+            ],
+        )
+        result = AgentEvalConverter.convert_message(msg)
+        tc = result[0]["content"][0]
+        assert tc["type"] == "tool_call"
+        assert "arguments" in tc
+        assert tc["arguments"] == {}
+
     def test_assistant_text_and_tool_call(self) -> None:
         msg = Message(
             "assistant",