pgsql: Fix EXPLAIN MERGE output when no tuples are processed

Fix EXPLAIN MERGE output when no tuples are processed

An 'else' clause was misplaced in commit 598ac10be1c2, making zero-rows
output look a bit silly.  Add a test case for it.

Pointed out by Tom Lane.

Discussion: https://postgr.es/m/21030.1652893083@sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/12e423e21d8ef47d95a099c12f625f6d191eaf92

Modified Files
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src/backend/commands/explain.c      | 27 +++++++++++++++------------
src/test/regress/expected/merge.out | 20 +++++++++++++++++++-
src/test/regress/sql/merge.sql      |  8 +++++++-
3 files changed, 41 insertions(+), 14 deletions(-)