fix: guard against None converter results in RemoteA2aAgent

Converter functions (convert_a2a_message_to_event, convert_a2a_task_to_event)
return Optional[Event], but several handler paths in RemoteA2aAgent access
attributes on the result without checking for None first. This causes
AttributeError when a converter returns None (e.g., for empty messages
or metadata-only events).

Add `if not event: return None` guards at all four call sites in both
the legacy and new-style response handlers.

Github-Issue: #5187

Author: enjoykumawat

src/google/adk/agents/remote_a2a_agent.py

@@ -476,6 +476,8 @@ async def _handle_a2a_response(
           event = convert_a2a_message_to_event(
               update.status.message, self.name, ctx, self._a2a_part_converter
           )
+          if not event:
+            return None
           if event.content is not None and update.status.state in (
               TaskState.submitted,
               TaskState.working,
@@ -501,6 +503,8 @@ async def _handle_a2a_response(
           # for now.
           return None
 
+        if not event:
+          return None
         event.custom_metadata = event.custom_metadata or {}
         event.custom_metadata[A2A_METADATA_PREFIX + "task_id"] = task.id
         if task.context_id:
@@ -513,6 +517,8 @@ async def _handle_a2a_response(
         event = convert_a2a_message_to_event(
             a2a_response, self.name, ctx, self._a2a_part_converter
         )
+        if not event:
+          return None
         event.custom_metadata = event.custom_metadata or {}
 
         if a2a_response.context_id:
@@ -583,6 +589,8 @@ async def _handle_a2a_response_v2(
         event = self._config.a2a_message_converter(
             a2a_response, self.name, ctx, self._config.a2a_part_converter
         )
+        if not event:
+          return None
         event.custom_metadata = event.custom_metadata or {}
 
         if a2a_response.context_id: