fix: stop GetIP erroring when the IP flag has a nil default

[leno23]

Summary

Fixes #478
Fixes #351

When an IP flag defaults to nil, GetIP round-trips through Value.String() and receives "<nil>", which ipConv rejected. Set("") already treats empty input as "no IP"; this mirrors that on the read side by returning (net.IP(nil), nil) for "" and "<nil>".

Test plan

• Added TestGetIPNilDefault covering IP and IPVar with a nil default
• go test ./...

Made with Cursor

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[tomasaschan]

@leno23 if you don't sign the CLA I will implement automation that automatically closes any PRs from you. Please stop spamming.