Avoid emitting empty ASR config

When no STT vendor is configured, stop serializing an empty asr object. This prevents the generated Asr model from defaulting the vendor back to ares and keeps ASR fallback behavior owned by the API.

Author: seymourtang

src/agora_agent/agentkit/agent_session.py

@@ -397,6 +397,9 @@ def _request_properties_for_start(
         resolved_preset: typing.Optional[str],
         pipeline_id: typing.Optional[str],
     ) -> typing.Any:
+        asr = resolved_properties.get("asr")
+        if isinstance(asr, dict) and "vendor" not in asr:
+            return resolved_properties
         try:
             return _start_properties_from_mapping(resolved_properties)
         except Exception as exc: