Use state if mirror branch regex is unknown in project mirror

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What does this MR do and why?

Fixes a bug with the gitlab_project_mirror resource where it would be stuck in a perpetual plan if the mirror_branch_regex attribute wasn't provided. Set to use state for unknown which exposed another bug where the regex wasn't getting checked for the empty string on update. This fixes both issues.

References

Resolves #6473 (closed)