Fix junit writing bug introduced in #16388

[mrwright]

#16388 introduced a bug where, with --junit-format=global, the junit file would indicate an error (with no message) even if everything passed. That was because _generate_junit_contents would check if messages_by_file was empty or not to determine if there were failures, but with --junit-format=global we'd pass in a dictionary of the form {None: all_messages}; all_messages would be empty, but the resulting dictionary wouldn't be.

The fix is to pass in an empty dictionary if there are no messages.

I've tested manually with --junit-format=global and --junit-format=per_file in the successful case to make sure the files are written correctly now.

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

discord.py (https://github.com/Rapptz/discord.py): typechecking got 1.08x faster (172.1s -> 158.9s)
(Performance measurements are based on a single noisy sample)