(PUP-1070) cleanup empty/invalid lockfiles

Currently lockfiles won't be cleaned up automatically if they're empty or contain
non numeric characters.

This commit solves this by verifying that the content of the lockfile
contains a number or otherwise return nil which will result in deleting
the pidfile.
Merge branch 'ticket/master/PUP-1070-cleanup-invalid-lockfiles'

* ticket/master/PUP-1070-cleanup-invalid-lockfiles:
  (PUP-1070) cleanup empty/invalid lockfiles