fix: Fix Type Error by initializing user_content as a Content object

PiperOrigin-RevId: 878682788

Author: google-genai-bot

src/google/adk/evaluation/evaluation_generator.py

@@ -280,7 +280,7 @@ def convert_events_to_eval_invocations(
     invocations = []
     for invocation_id, events in events_by_invocation_id.items():
       final_response = None
-      user_content = ""
+      user_content = Content(parts=[])
       invocation_timestamp = 0
       app_details = None
       if (