When using column type `inet` Rails will silently return the value
as `nil` when the value is not valid according to its type cast
using `IpAddr`. It's not very user friendly to return an error
"IP Address can't be blank" when a value was clearly given but
was not the right format. This validator will look at the value
before Rails type casts it when the value itself is `nil`.
This enables the validator to return a specific and useful error
message.