Fix the HTML table with null text in span cells that don't show correctly

[whoisltd]

Fixes #1293

This Pr to fix HTML table with td tag have rowspan, colspan > 1 and text inside is null the pdf output not show table correctly

Checklist:

• [x] A unit test is covering the code added / modified by this PR

• [ ] This PR is ready to be merged

• [ ] In case of a new feature, docstrings have been added, with also some documentation in the docs/ folder

• [ ] A mention of the change is present in CHANGELOG.md

By submitting this pull request, I confirm that my contribution is made under the terms of the GNU LGPL 3.0 license.

[whoisltd]

That's well done, thank you for your contribution 👍

Could you please add a mention of this fix in CHANGELOG.md as part of this PR?

yep, I added it to CHANGELOG.md