fix: allow nil default IP flags in GetIP

[happysnaker]

Summary

• treat the zero-value string form of a nil net.IP flag as a nil result in GetIP()
• keep IPVar(..., nil, ...) usable for optional IP flags without forcing callers through parse errors
• add regression coverage for nil default IP flags

Motivation

When an IP flag is declared with a nil default value, the internal string form becomes <nil>. GetIP() currently passes that through ipConv, which tries to parse it as a literal IP string and returns an error. This makes optional IP flags awkward to model when callers want an actual nil default.

This change keeps the existing parsing behavior for user-provided values, but lets untouched nil defaults round-trip cleanly through GetIP().

Closes #351

[CLAassistant]

All committers have signed the CLA.

[happysnaker]

Quick status update: the CLA check is green now, so the PR is no longer blocked on contributor-signature state.

Current intent stays the same: make nil-default IP flags round-trip cleanly through GetIP() instead of failing on the internal <nil> string form.