fixed chunk_overlap is None (#7703)

api/core/rag/index_processor/index_processor_base.py

 
             character_splitter = FixedRecursiveCharacterTextSplitter.from_encoder(
                 chunk_size=segmentation["max_tokens"],
-                chunk_overlap=segmentation.get('chunk_overlap', 0),
+                chunk_overlap=segmentation.get('chunk_overlap', 0) or 0,
                 fixed_separator=separator,
                 separators=["\n\n", "。", ". ", " ", ""],
                 embedding_model_instance=embedding_model_instance