Save keywords to cache only when it's no empty

lightrag/operate.py

@@ -760,7 +760,7 @@ async def extract_keywords_only(
     # 6. Parse out JSON from the LLM response
     match = re.search(r"\{.*\}", result, re.DOTALL)
     if not match:
-        logger.error("No JSON-like structure found in the result.")
+        logger.error("No JSON-like structure found in the LLM respond.")
         return [], []
     try:
         keywords_data = json.loads(match.group(0))
@@ -772,6 +772,7 @@ async def extract_keywords_only(
     ll_keywords = keywords_data.get("low_level_keywords", [])
 
     # 7. Cache only the processed keywords with cache type
+    if hl_keywords or ll_keywords:
         cache_data = {"high_level_keywords": hl_keywords, "low_level_keywords": ll_keywords}
         await save_to_cache(
             hashing_kv,