core: taskcleaner.sh should treat null argument as -h

Adding a check for the USERNAME which is mandatory.

Change-Id: I8505ff75d1b22eb27b3aa928d254cc6830b9f4c5
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=924246
Signed-off-by: Eli Mesika <emesika@redhat.com>