GitLabPatchesCustomField: check array indices before use

Brennen Bearnes requested to merge work/robustify-mr-widget into wmf/stable

We never want a situation where an unexpected return value from the GitLab search API throws an exception that renders the rest of the page un-viewable. This should at least be a bit more robust.

I note that isset() here would also return false on encountering a PHP null, but I believe that'd also indicate a weird return situation.

Also makes all early returns from the data-fetching function arrays, instead of mixing type.

Bug: T332607

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