commons/table/to-grid does not account for rowspan=0

[straker]

Noticed this while working on #2545. According to MDN, rowspan=0 means the row spans to the end of the table (tested in Chrome).

if its value is set to 0, it extends until the end of the table section (<thead>, <tbody>, <tfoot>, even if implicitly defined), that the cell belongs to

Our get-headers code will also need to be adjusted to handle rospawn of 0.

[padmavemulapati]

Verified in the axe-core latest develop code:
with the code snippet:
<!-- rowspan=0 - headers-->
<table>
<tr><td rowspan="0"></td><th scope="col">1</th><th scope="col" id="t1">2</th></tr>
<tr><th scope="row" id="t2"></th><td id="target"></td></tr>
</table>
on chrome:

on firefox: