Crashing the system process is inadvisable

When asking for the set of services published by a package, it's
quite possible that there are none, in which case the returned List<>
is null rather than valid-but-empty.  Don't bother looking at it
when it's null.

Bug 23614440

Change-Id: Ibebb26b9c3f75ec810a95f1b9d2663e884cb98bc