Don't propagate a null subtransaction snapshot up to parent transaction.

This oversight could cause logical decoding to fail to decode an outer
transaction containing changes, if a subtransaction had an XID but no
actual changes.  Per bug #14279 from Marko Tiikkaja.  Patch by Marko
based on analysis by Andrew Gierth.

Discussion: <20160804191757.1430.39011@wrigleys.postgresql.org>