filter out /dev/null entries in list of patch files for PR

Ran into this issue when testing https://github.com/hpcugent/easybuild-easyconfigs/pull/1217, see https://github.com/hpcugent/easybuild-easyconfigs/pull/1217.diff
OK, I looked into this a bit more, the `/dev/null` bit not specified to git/GitHub.

Since `det_patched_files` only checks the `+++` lines, a `/dev/null` will indicate a *removed* file, not a new file.

I'll filter out the `/dev/null` entries in `det_patches_files` directly, you're right.