IMPALA-935: Make PlanFragment.getDestFragment() return null if no destination is set.

Change-Id: I269a7f552d7ff67ff4d65e86e8c6df9c41d0fca1
Reviewed-on: http://gerrit.ent.cloudera.com:8080/2153
Reviewed-by: Alex Behm <alex.behm@cloudera.com>
Tested-by: jenkins