Bug 31202: Don't remove optional SIP fields with a value of "0"

Bug 31202: Don't remove optional SIP fields with a value of "0"

If the value of a SIP field is "0", that evaluates to false, so any calls to maybe_add with a value of "0" will not get added to the SIP response message.

Test Plan:
1) Apply this patch
2) prove t/db_dependent/SIP/Message.t

Signed-off-by: Michal Urban <michalurban177@gmail.com>
JK: Adjust commit title

Signed-off-by: Joonas Kylmälä <joonas.kylmala@iki.fi>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>