Commit 5a3574d7 added the test for decoding of two-phase transactions
during the build of a consistent snapshot. The test forgot to skip empty
xacts which can lead to decoding of extra empty transactions due to
background activity by autovacuum.

Per report by buildfarm.

Reported-by: Tom Lane
Discussion: https://postgr.es/m/363512.1610171267@sss.pgh.pa.us