Avoid uninitialized value error in TAP tests' Cluster->psql

If the method is called in scalar context and we didn't pass in a stderr
handle, one won't be created. However, some error paths assume that it
exists, so in this case create a dummy stderr to avoid the resulting
perl error.

Per gripe from Oleg Tselebrovskiy <o.tselebrovskiy@postgrespro.ru> and
adapted from his patch.

Discussion: https://postgr.es/m/378eac5de4b8ecb5be7bcdf2db9d2c4d@postgrespro.ru

src/test/perl/PostgreSQL/Test/Cluster.pm

@@ -2199,6 +2199,14 @@ sub psql
 			$ret = $?;
 		};
 		my $exc_save = $@;
+
+		# we need a dummy $stderr from hereon, if we didn't collect it
+		if (! defined $stderr)
+		{
+			my $errtxt = "<not collected>";
+			$stderr = \$errtxt;
+		}
+
 		if ($exc_save)
 		{