IPv4 parser and the empty string as input

[triple-underscore]

https://url.spec.whatwg.org/#concept-ipv4-parser

It is unclear what happen if the empty string is passed to the IPv4 parser as input.
The step 3 makes the "parts" list empty, and the algorithm gets stuck at the step 11.
(Probably, the algorithm should return the same input. )

[annevk]

This is a duplicate of #79 in a way, but I'll leave it open for now.

There's one question here as I see it:

1. Should other standards be able to directly invoke the IPv4 parser? (Inclined to say no.)

If they cannot, then it's simply a matter of making sure that either IPv4 handles it or the host parser never invokes it with the empty string.

[annevk]

I have a fix for #79 in #189. Marking this as a duplicate since it appears to be a concrete one after all.