Fix ValueError when processing tables with empty cells

[inacionery]

Summary

This PR fixes a ValueError: min() arg is an empty sequence that occurred when processing PDFs with tables that have empty or invalid cell sequences.

Problem

When to_markdown() processes a table with no valid cells, accessing the t.bbox property causes PyMuPDF's table module to call min() on an empty sequence, resulting in a crash.

Error trace:

File "pymupdf4llm/helpers/pymupdf_rag.py", line 1057, in get_page_output
    omitted_table_rects.append(pymupdf.Rect(t.bbox))
                                            ^^^^^^
File "pymupdf/table.py", line 1506, in bbox
    min(map(itemgetter(0), c)),
    ^^^^^^^^^^^^^^^^^^^^^^^^^^
ValueError: min() arg is an empty sequence