Raise ArgumentError if IP is nil

[ghg]

We end up doing a lot of calculations before actually instantiating an IPAddress object. Occasionally we end up passing a nil value which results in:

pry(main)> ip = IPAddress::IPv4.new(nil)
NoMethodError: undefined method `split' for nil:NilClass

This PR raises the same exception that is raised for invalid input when given nil input.

[ghg]

After submitting, I noticed my editor stripped a lot of extraneous whitespace. I'll be more than happy to split this into a separate commit, or undo the whitespace deletion all together.

Thanks!

[mikemackintosh]

@ghg could you please resubmit without the whitespace changes?

Looking at the following looks good and consistent across IPv4 and 6:
ghg@86a7876?w=0

[ghg]

My apologies for missing the notification on this. I have resubmitted the PR without the whitespace changes ( #64 ).

Thanks!