[fixes #92] If phone=0 make it a null in the DB.

Fixes #92.

~~If the phone number for establishment from the Durham API is zero, save it into our DB as Null instead.~~

* ~~makes the phone number nullable in the model~~
* ~~clean code in the form~~

~~Database needs to be migrated or adjusted manually (that's what I did).~~

~~ALTER TABLE inspections_establishment ALTER COLUMN phone_number DROP NOT NULL;~~
~~UPDATE inspections_establishment SET phone_number=Null WHERE phone_number='0';~~