Fix: "AttributeError(\"'list' object has no attribute 'get'\")" (#12518)

### What problem does this PR solve?
https://github.com/infiniflow/ragflow/issues/12515

### Type of change

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

rag/prompts/generator.py

@@ -38,6 +38,8 @@ def get_value(d, k1, k2):
 
 
 def chunks_format(reference):
+    if not reference or (reference is not dict):
+        return []
     return [
         {
             "id": get_value(chunk, "chunk_id", "id"),