I submitted a pull request that fixes this at https://github.com/openwebwork/pg/pull/223.
Now all of the cell options are initialized. Warnings like this would have been thrown out all over the place when I first wrote this macro, so I'm guessing maybe something changed with how Perl behaves on those comparisons. Or how WeBWorK responds to getting such warnings from Perl.
With the patch, both your MWE and a big file I have for testing the features of niceTable.pl are working as expected.
If you would like this sooner than the normal workflow provides (approval of the pull request into the develop branch, followed by the next release of pg) you can just copy that niceTables.pl from my GitHub account by following the pull request.
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niceTables: Use of uninitialized value in numeric eq
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