scripts/doc_to_xls.py: fix spreadsheet generation

The logic which sets the max_length is wrong: it shall always use
sheet[0], as this may be the only row on a table.

Yet, it might be possible that a caller to test_to_xls() would
have been sending a completely empty sheet. While this doesn't
occur currently, it doesn't hurt adding an explicit check,
reporting a warning if this is indeed the case.

Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Reviewed-by: Kamil Konieczny <kamil.konieczny@linux.intel.com>