### What problem does this PR solve?
fixed /datasets/retrieval API:
KeyError('size') and 'doc_ids': ['Field may not be null.']
### Type of change
- [x] Bug Fix (non-breaking change which fixes an issue)
### What problem does this PR solve?
fixed /datasets/retrieval API:
KeyError('size') and 'doc_ids': ['Field may not be null.']
### Type of change
- [x] Bug Fix (non-breaking change which fixes an issue)