### What problem does this PR solve?

fix "no `tc` element at grid_offset", just log warning and ignore.
stacktrace:
```
Traceback (most recent call last):
  File "/ragflow/rag/svr/task_executor.py", line 620, in handle_task
    await do_handle_task(task)
  File "/ragflow/rag/svr/task_executor.py", line 553, in do_handle_task
    chunks = await build_chunks(task, progress_callback)
  File "/ragflow/rag/svr/task_executor.py", line 257, in build_chunks
    cks = await trio.to_thread.run_sync(lambda: chunker.chunk(task["name"], binary=binary, from_page=task["from_page"],
  File "/ragflow/.venv/lib/python3.10/site-packages/trio/_threads.py", line 447, in to_thread_run_sync
    return msg_from_thread.unwrap()
  File "/ragflow/.venv/lib/python3.10/site-packages/outcome/_impl.py", line 213, in unwrap
    raise captured_error
  File "/ragflow/.venv/lib/python3.10/site-packages/trio/_threads.py", line 373, in do_release_then_return_result
    return result.unwrap()
  File "/ragflow/.venv/lib/python3.10/site-packages/outcome/_impl.py", line 213, in unwrap
    raise captured_error
  File "/ragflow/.venv/lib/python3.10/site-packages/trio/_threads.py", line 392, in worker_fn
    ret = context.run(sync_fn, *args)
  File "/ragflow/rag/svr/task_executor.py", line 257, in <lambda>
    cks = await trio.to_thread.run_sync(lambda: chunker.chunk(task["name"], binary=binary, from_page=task["from_page"],
  File "/ragflow/rag/app/naive.py", line 384, in chunk
    sections, tables = Docx()(filename, binary)
  File "/ragflow/rag/app/naive.py", line 230, in __call__
    while i < len(r.cells):
  File "/ragflow/.venv/lib/python3.10/site-packages/docx/table.py", line 438, in cells
    return tuple(_iter_row_cells())
  File "/ragflow/.venv/lib/python3.10/site-packages/docx/table.py", line 436, in _iter_row_cells
    yield from iter_tc_cells(tc)
  File "/ragflow/.venv/lib/python3.10/site-packages/docx/table.py", line 424, in iter_tc_cells
    yield from iter_tc_cells(tc._tc_above)  # pyright: ignore[reportPrivateUsage]
  File "/ragflow/.venv/lib/python3.10/site-packages/docx/oxml/table.py", line 741, in _tc_above
    return self._tr_above.tc_at_grid_offset(self.grid_offset)
  File "/ragflow/.venv/lib/python3.10/site-packages/docx/oxml/table.py", line 98, in tc_at_grid_offset
    raise ValueError(f"no `tc` element at grid_offset={grid_offset}")
ValueError: no `tc` element at grid_offset=10
```

### Type of change

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