Filterx invalid utf8 fix for unset_empties

[bshifter]

• pre-validation of utf8 input string on eval
• targets literal generator list parse-time utf8 validation

[alltilla]

Shouldn't we also replace the compare function to be g_utf8_collate even for ignorecase=false? Thinking about the different ways an utf-8 string can be represented but have the same meaning.

The merge-base changed after approval.

[alltilla]

Please rebase to main to drop the first commit.

[alltilla]

Let's talk about UTF-8 IRL, this PR is good for now.

The merge-base changed after approval.