CI: Fixup failure to get changed files on merge

The checkout might be too shallow so the before commit isn't available.
The logic was already written to take this into account (it then generates
an empty 'changed.txt' which falls back to testing everything), but the
error code would still force terminate the job.

Hopefully we can find a way to make the logic work for merge events too in
the future, but for now this is a quick fix.