Fixed #22602 -- Improve code coverage of makemigrations command tests

Brings coverage of `makemigrations` up to 98% from 61%. Three lines are left uncovered. The first requires adding another app so that you can specify two app labels. The last two are actually unreachable as far as I can tell. A `ValueError` can only be reached if you call `max([])`, which won't ever happen inside `handle_merge`.

Thanks claudep for the report and Tim Graham for the review.