test: Robustify check-storage-msdos test
test: Robustify check-storage-msdos test

A classless `<tbody>` causes `getAttribute("class")` to return a null
value instead of an empty string in chromium, which cannot be used with
`in`. Provide an empty string fallback instead.

Reviewed-by: Peter <a316049dd34607c62d4611d63ab22a6454add223@redhat.com>

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