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

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