Deal with apps known to LocalTransport backend but with no data

An app might have backed something up and later deleted it, in which
case it will have a directory in the LocalTransport bookkeeping but
with no actual dataset.  Skip these packages at restore time, since
we know a priori that there will be nothing to do.

Change-Id: I85853097bc65ed60e5efd217ee6553b50459e445