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
agree with @wpoely86 why print it before the filter? (esp if nothing changed in 99% of the cases)