pgsql: Fix poll_query_until() timeout handling with an undefined query

Fix poll_query_until() timeout handling with an undefined query in Cluster.pm

On timeout, poll_query_until() would fail to print its report when
"$query" was undefined. Instead of the expected report, this prevented
the contents of stdout and stderr from being shown.

Note that some of the in-core tests use an undefined query for
connection-only polls.

Author: Bryan Green <dbryan(dot)green(at)gmail(dot)com>
Reviewed-by: Jonathan Gonzalez V. <jonathan(dot)abdiel(at)gmail(dot)com>
Discussion: https://postgr.es/m/0f515c6d-6032-4c5f-80ac-5c78faae9522@gmail.com
Backpatch-through: 14

Branch
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REL_16_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/37c4a0730f8e9230b8e15a33ec5ebd4dae24ca96

Modified Files
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src/test/perl/PostgreSQL/Test/Cluster.pm | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)