Fix: Enforce default embedding model in create_dataset / update_dataset (#8486)

### What problem does this PR solve?

Previous:
- Defaulted to hardcoded model 'BAAI/bge-large-zh-v1.5@BAAI'
- Did not respect user-configured default embedding_model

Now:
- Correctly prioritizes user-configured default embedding_model

Other:
- Make embedding_model optional in CreateDatasetReq with proper None
handling
- Add default embedding model fallback in dataset update when empty
- Enhance validation utils to handle None values and string
normalization
- Update SDK default embedding model to None to match API changes
- Adjust related test cases to reflect new validation rules

### Type of change

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

sdk/python/ragflow_sdk/ragflow.py

@@ -53,7 +53,7 @@ class RAGFlow:
         name: str,
         avatar: Optional[str] = None,
         description: Optional[str] = None,
-        embedding_model: Optional[str] = "BAAI/bge-large-zh-v1.5@BAAI",
+        embedding_model: Optional[str] = None,
         permission: str = "me",
         chunk_method: str = "naive",
         parser_config: Optional[DataSet.ParserConfig] = None,