fix empty content issue

Author: daixba

src/api/models/bedrock.py

@@ -277,7 +277,7 @@ def _parse_messages(self, chat_request: ChatRequest) -> list[dict]:
                     }
                 )
             elif isinstance(message, AssistantMessage):
-                if message.content:
+                if message.content.strip():
                     # Text message
                     messages.append(
                         {