Consistently return array from bulk lookup, even if translation(s) missing

[sambostock]

In the event of we fail to lookup one or more keys during a bulk lookup, we should still return an array of translation results.

Currently, doing a bulk lookup for multiple keys returns only the "translation missing" string for the first key.

• The first commit adds a test describing the existing behavior.
• The second commit fixes it and updates the test. All other tests appear unaffected.

[radar]

Thank you!