Strengthen tests for 5ef34a8fc3

The test query in 5ef34a8fc3 is running over the empty emp1 table giving the
same (empty) return both with and without the fix.  Add one row to that table
to make not just the test query plan, but also the test query result different.

Reported-by: Richard Guo
Bug: #18261
Discussion: https://postgr.es/m/CAMbWs49igjcszLgicb4D1N21_5iNDoxheJ7KFmAcs_z%3DLx6jhg%40mail.gmail.com

src/test/regress/expected/join.out

@@ -6881,6 +6881,7 @@ WHERE t1.id = emp1.id RETURNING emp1.id, emp1.code;
 (3 rows)
 
 -- Check that SJE does not mistakenly omit qual clauses (bug #18187)
+insert into emp1 values (1, 1);
 explain (costs off)
 select 1 from emp1 full join
     (select * from emp1 t1 join

src/test/regress/sql/join.sql

@@ -2623,6 +2623,7 @@ UPDATE emp1 SET code = t1.code + 1 FROM t1
 WHERE t1.id = emp1.id RETURNING emp1.id, emp1.code;
 
 -- Check that SJE does not mistakenly omit qual clauses (bug #18187)
+insert into emp1 values (1, 1);
 explain (costs off)
 select 1 from emp1 full join
     (select * from emp1 t1 join