pgsql: Fix the test for decoding of two-phase transactions.

Fix the test for decoding of two-phase transactions.

Commit 5a3574d7b3 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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/e33d004900f76c35759293fdedd4861b198fbf5b

Modified Files
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contrib/test_decoding/expected/twophase_snapshot.out | 4 ++--
contrib/test_decoding/specs/twophase_snapshot.spec   | 2 +-
2 files changed, 3 insertions(+), 3 deletions(-)