[PATCH] Avoid uninitialized-value error in poll_query_until timeout diagnostic

Greetings,

PostgreSQL::Test::Cluster::poll_query_until() accepts an undefined query
for connection-only checks -- several recovery TAP tests call it that way:

$node->poll_query_until('postgres', undef, '');

On timeout it builds a diag message that interpolates $query:

diag qq(poll_query_until timed out executing this query:
$query
...);

Cluster.pm runs under "use warnings FATAL => 'all'", so when $query is
undefined this does not warn, it dies with "Use of uninitialized value
$query in concatenation". The result is that a timeout in a connection
only poll fails with an uninitialized-value error instead of printing
the timeout diagnostic the code is trying to produce, hiding the actual
failure.

$ perl -e 'use warnings FATAL => "all"; my $q; my $s = qq(q: $q);'
Use of uninitialized value $q in concatenation (.) or string at -e
line 1.

The fix uses a fallback string when the query is undefined:

my $msg_query = $query // '(undef - connection attempt only)';

and interpolates $msg_query instead. Test-only, one line.

--
Bryan Green
EDB: https://www.enterprisedb.com