To avoid assertion while no rows in excel (#197)

### What problem does this PR solve?

_Briefly describe what this PR aims to solve. Include background context
that will help reviewers understand the purpose of the PR._

Issue link:#[[Link the issue
here](https://github.com/infiniflow/ragflow/issues/196)]

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)
- [ ] New Feature (non-breaking change which adds functionality)
- [ ] Breaking Change (fix or feature that could cause existing
functionality not to work as expected)
- [ ] Documentation Update
- [ ] Refactoring
- [ ] Performance Improvement
- [ ] Test cases
- [ ] Python SDK impacted, Need to update PyPI
- [ ] Other (please describe):

rag/app/table.py

@@ -40,6 +40,7 @@ class Excel(ExcelParser):
         for sheetname in wb.sheetnames:
             ws = wb[sheetname]
             rows = list(ws.rows)
+            if not rows:continue
             headers = [cell.value for cell in rows[0]]
             missed = set([i for i, h in enumerate(headers) if h is None])
             headers = [