fix(rule): Prevent th-has-data-cells from crashing on empty rows

[WilcoFiers]

Since rows can be empty (<tr> </tr>) getScope shouldn't assume there are cells in every row.

Closes #1266

Reviewer checks

Required fields, to be filled out by PR reviewer(s)

• Follows the commit message policy, appropriate for next version
• Has documentation updated, a DU ticket, or requires no documentation change
• Includes new tests, or was unnecessary
• Code is reviewed for security by: << Marcy Sutton >>

[marcysutton]

It's probably fine this time, but it would help to have "Closes #" in the body of your commit so it ends up in the changelog. (I only saw it in the PR description)

[WilcoFiers]

@marcysutton that message is in the body of the PR, so it'll be included when I squash I think.