Use fallback approval rule if no eligible rules exist

What does this MR do?

Modifies use_fallback? logic. If no regular rules with approvers exist, then fallback approval rule is used

Closes: https://gitlab.com/gitlab-org/gitlab-ce/issues/61044

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content