Fix typos CI job for PRs that don't edit any files or do edit binary files

[LilithHafner]

Fixes #52599.

Two issues, each fixed by its own commit

1. mis-handling empty sets of "new" and "old" files (e.g. if a PR exclusively adds files, we considered the whole repo's typos to be false positives rather than assuming no false positives)
2. crashing on binary files

These two combined cause crashing on any PR that exclusively adds or exclusively removes files.

The github history is a hot mess but the git history should be pristine.

[LilithHafner]

Ideally, I'd like to merge this PR before #45641 so that we can see the fix in the wild and get an extra confirmation that it works.