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
also use `filter` like
```python
patched_files=filter('/dev/null'.__ne__, patched_files)
```