pgsql: Strengthen tests for 5ef34a8fc3

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

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/bea18b1c949145ba2ca79d4765dba3cc9494a480

Modified Files
--------------
src/test/regress/expected/join.out | 1 +
src/test/regress/sql/join.sql      | 1 +
2 files changed, 2 insertions(+)