fix: Treat empty-but-defined text responses as a text response

Handles an edge case in structured type generation where the JSON content was previously completed but a final, empty response is sent with a STOP finish reason.

PiperOrigin-RevId: 774917500

Author: google-genai-bot

src/google/adk/models/google_llm.py

@@ -124,7 +124,7 @@ async def generate_content_async(
         if (
             llm_response.content
             and llm_response.content.parts
-            and llm_response.content.parts[0].text
+            and llm_response.content.parts[0].text is not None
         ):
           part0 = llm_response.content.parts[0]
           if part0.thought: