email: Fix userdb lookups with LDAP
Without the base attribute set, LDAP search returns no results and because of the lookup failure LMTP will not be able to deliver message even though postfix has successfully received the messages.
Tests:
- On a fresh container, send a message to a local user using roundcube. The user should receive the message successfully.
Signed-off-by: Sunil Mohan Adapa sunil@medhas.org