Always generate lookup tables, even if they're empty

If we don't generate tables even when they're going to be empty, then we have to
figure out how to remove the 'transport_maps' and 'smtp_sasl_password_maps'
config lines that point at them. Postfix is fine with empty tables, so this is
the simplest way to solve the problem of undefined lookup mappings.

The alternative is to set the 'transport_maps' and 'smtp_sasl_password_maps'
properties to nothing, which Postfix is also fine with, or remove them entirely.