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

contrib/test_decoding/expected/twophase_snapshot.out

@@ -22,14 +22,14 @@ step s1init: <... completed>
 
 init           
 step s1insert: INSERT INTO do_write DEFAULT VALUES;
-step s1start: SELECT data  FROM pg_logical_slot_get_changes('isolation_slot', NULL, NULL, 'include-xids', 'false', 'two-phase-commit', '1');
+step s1start: SELECT data  FROM pg_logical_slot_get_changes('isolation_slot', NULL, NULL, 'include-xids', 'false', 'skip-empty-xacts', '1', 'two-phase-commit', '1');
 data           
 
 BEGIN          
 table public.do_write: INSERT: id[integer]:2
 COMMIT         
 step s2cp: COMMIT PREPARED 'test1';
-step s1start: SELECT data  FROM pg_logical_slot_get_changes('isolation_slot', NULL, NULL, 'include-xids', 'false', 'two-phase-commit', '1');
+step s1start: SELECT data  FROM pg_logical_slot_get_changes('isolation_slot', NULL, NULL, 'include-xids', 'false', 'skip-empty-xacts', '1', 'two-phase-commit', '1');
 data           
 
 BEGIN          

contrib/test_decoding/specs/twophase_snapshot.spec

@@ -16,7 +16,7 @@ session "s1"
 setup { SET synchronous_commit=on; }
 
 step "s1init" {SELECT 'init' FROM pg_create_logical_replication_slot('isolation_slot', 'test_decoding');}
-step "s1start" {SELECT data  FROM pg_logical_slot_get_changes('isolation_slot', NULL, NULL, 'include-xids', 'false', 'two-phase-commit', '1');}
+step "s1start" {SELECT data  FROM pg_logical_slot_get_changes('isolation_slot', NULL, NULL, 'include-xids', 'false', 'skip-empty-xacts', '1', 'two-phase-commit', '1');}
 step "s1insert" { INSERT INTO do_write DEFAULT VALUES; }
 
 session "s2"