fix: handle empty response in forced rewrite function

- Updated the `_forced_rewrite` function to strip whitespace from the extracted text and added a warning log if the response is empty, preventing potential issues with empty rewrites.

surfsense_backend/app/agents/new_chat/tools/update_memory.py

@@ -189,8 +189,11 @@ async def _forced_rewrite(content: str, llm: Any) -> str | None:
             [HumanMessage(content=prompt)],
             config={"tags": ["surfsense:internal"]},
         )
-        text = extract_text_content(response.content)
-        return text.strip()
+        text = extract_text_content(response.content).strip()
+        if not text:
+            logger.warning("Forced rewrite returned empty text; aborting rewrite")
+            return None
+        return text
     except Exception:
         logger.exception("Forced rewrite LLM call failed")
         return None