merge_pr_44361

Add null-check to aria_utils verifyLabelsBySelector (#44361)

• Add null-check to aria_utils verifyLabelsBySelector

Before this commit, verifyLabelsBySelector calls test_driver.get_computed_label
and then does string-manipulation on the result. For some reason, this
get_computed_label invocation returns null in Firefox, and so the
string-manipulation causes a JS exception to be thrown, which makes for awkward
test failure results.

Let's just explicitly assert that the result is not-null, to make that
expectation clearer and to give a cleaner test-failure in cases where it is
unexpectedly null.