Backport of '18.8.0: Merge Request reviewer dropdown crashes and does not send request'

What does this MR do and why?

Backports the fix in !219176 (commits) to 18.8 when the regression was introduced via Disable Agents in MR assignment when out of cre... (!216368 - merged)

Regression report: 18.8.0: Merge Request reviewer dropdown crashes and does not send request

When in single selection mode, the collapsible listbox returns a single item not wrapped in an array. In the rest of this feature, we expect it to be wrapped in an array, even in the case of there only being a single reviewer.

As part of this fix, correct the unit tests to occasionally emit a single string, as is the case in the non-multiselect version.

References

• Regression introduced via: !216368 (merged)
• Bug/regression reported: #586421 (comment 3013103092)
• 18.9 fix MR: !219176 (commits)

Note: no conflicts when cherry-picking

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
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• Ensure the e2e:test-on-omnibus-ee job has succeeded, or if it has failed, investigate the failures. If you determine the failures are unrelated, you may proceed. If you need assistance investigating, reach out to a Software Engineer in Test in #s_developer_experience.

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