For Subnet gateway_ip, pass None for empty string

A None gateway_ip has special meaning for create_subnet but
properties sanitise None values to be empty strings.

With this change, the following properties should all result
in gateway_ip=None being passed to create_subnet:
gateway_ip: null
gateway_ip: ~
gateway_ip: ''

Change-Id: I234542f7f2d7bddf7dee01f8a9c56c021b5f0c8c
Closes-Bug: #1226666
Merge "For Subnet gateway_ip, pass None for empty string"