filter empty strings from ps -U 0

The script was not working for me without this fix:
```
>>> import subprocess
>>> ps = subprocess.Popen("ps -U 0", shell=True, stdout=subprocess.PIPE)
>>> parts = [line for line in ps.stdout.read().split('\n') if 'samba' in line]
>>> line = parts[0]
' 6411 pts/9    00:00:00 samba'
>>> line.split(' ')
['', '6411', 'pts/9', '', '', '', '00:00:00', 'samba']
>>> filter(None, _)
['6411', 'pts/9', '00:00:00', 'samba']
```