models: HostBib: Properly handle 774s without $w

Using `.empty?` still requires the value to not be `nil`.  There is
at least one record that has no `w` and therefore has a `nil` value;
using `.present?` checks for nil *and* empty values.

Additionally, ensure no `nil` values escape into the subfields list.

Fixes: LIBIT-10644
Ref: AP-778

Author: awilfox