Avoid setitng the CONN_STR in queues unless it is meant to be set (#13025)

Avoid setitng the CONN_STR in queues unless it is meant to be set (#13025)

Since the move to common leveldb and common redis the disk queue code
will check the connection string before defaulting to the DATADIR.

Therefore we should ensure that the connection string is kept empty
unless it is actually set.

Fix #13023

Signed-off-by: Andrew Thornton <art27@cantab.net>