Fix errors in coverage approval rule sync when either pipeline does not have a coverage

What does this MR do and why?

This fixes errors that occur when either the base pipeline or the head pipeline does not have a coverage.

When either of the pipeline does not have a coverage, the merge request is considered not approved, and still requires manual approval.

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How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

Related to #337108 (closed)