Code review comment for lp:~julian-edwards/maas/models-for-static-dhcp

I've made the IPAddress.mac_address nullable in the case where the IP is not for a Node.

Additionally, we should note that there is no sensible migration you can do to create a static range for DHCP, it requires admin intervention. The simplest thing to do is if there is no static range defined, then continue as before with only dynamic leases. But this will come later in some actual code!