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/manual.py

@@ -109,6 +109,7 @@ def chunk(filename, binary=None, from_page=0, to_page=100000,
     sections = [(txt, sec_ids[i], poss)
                 for i, (txt, _, poss) in enumerate(sections)]
     for (img, rows), poss in tbls:
+        if not rows:continue
         sections.append((rows if isinstance(rows, str) else rows[0], -1,
                          [(p[0] + 1 - from_page, p[1], p[2], p[3], p[4]) for p in poss]))