pgsql: Fix the logical streaming test.

Fix the logical streaming test.

Commit 7259736a6e added the capability to stream changes in ReorderBuffer
which has some tests to test the streaming mode. It is quite possible that
while this test is running a parallel transaction could be logged by
autovacuum. Such a transaction won't perform any insert/update/delete to
non-catalog tables so will be shown as an empty transaction. Fix it by
skipping the empty transactions during this test.

Per report by buildfarm.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/82a0ba7707e010a29f5fe1a0020d963c82b8f1cb

Modified Files
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contrib/test_decoding/expected/stream.out | 4 ++--
contrib/test_decoding/sql/stream.sql      | 4 ++--
2 files changed, 4 insertions(+), 4 deletions(-)