core: user who added external events does not...

user who added external events does not appear in UI

Actually this will occurs also to VMs Hosts & Templates
Fixing code to get the Name in case that the User/VM/Host/Template id is
not null.

Change-Id: I750fb1873d730c14ddab761062c3ec85157f0a77
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=949557
Signed-off-by: Eli Mesika <emesika@redhat.com>