core: Potential NullPointerException at Entities.ObjectNames method

The method accepts a list that could contain null elements.
In case one or more of the elements in the list is null, an exception will be thrown.

Change-Id: If282b7a525800ffcb9a6f73292f01de3fca810d0
Bug-Url: https://bugzilla.redhat.com/1063173
Signed-off-by: Kobi Ianko <kobi@redhat.com>