Fix: Add a basic example when the example of content_tagging is empty (#6276)

### What problem does this PR solve?

When using LLM for auto-tag, if there are no examples, the tag format
generated by LLM may be wrong. This will cause Elasticsearch insert
errors. Adding basic examples can avoid this problem.

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

rag/svr/task_executor.py

@@ -359,6 +359,8 @@ async def build_chunks(task, progress_callback):
             cached = get_llm_cache(chat_mdl.llm_name, d["content_with_weight"], all_tags, {"topn": topn_tags})
             if not cached:
                 picked_examples = random.choices(examples, k=2) if len(examples)>2 else examples
+                if not picked_examples:
+                    picked_examples.append({"content": "This is an example", TAG_FLD: {'example': 1}})
                 async with chat_limiter:
                     cached = await trio.to_thread.run_sync(lambda: content_tagging(chat_mdl, d["content_with_weight"], all_tags, picked_examples, topn=topn_tags))
                 if cached: