core : Fix NPE in CommandBase buildCommandEntity

When execution context is null CommandBase
buildCommandEntity throws a NPE.

Change-Id: I6f6a2b7e0727a22a9649d0b305000f59eb4236a1
Bug-Url: https://bugzilla.redhat.com/1147971
Signed-off-by: Ravi Nori <rnori@redhat.com>