[TalhaHunter101]

Fixes #471

What kind of change does this PR introduce?

• Bug fix
• Feature
• Documentation update
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

Description

In index.js, normalizeIP assumed that ip is always a valid string. When req.ip is undefined (which can occur on Unix domain sockets where socket.remoteAddress is undefined, mock/injected requests, or aborted connections):

• !isIPv6(undefined) evaluated to true.
• ip.toLowerCase() threw TypeError: Cannot read properties of undefined (reading 'toLowerCase').
• Fastify crashed the incoming request with an unhandled 500 error.

This PR:

• Adds an early check if (typeof ip !== 'string') return '' in normalizeIP so missing or non-string IPs safely return an empty string key rather than throwing.
• Adds unit and integration tests verifying that normalizeIP(undefined) and requests with req.ip === undefined are handled gracefully without errors.

Checklist

• run npm test
• tests and/or benchmarks are included
• commit message and code follows the Developer's Certification of Origin (DCO)
• code follows the Code of Conduct

[SameRainbows]

Tested locally on Node 22 and verified across the test suite.

Key points verified:

• In Fastify, req.ip can resolve to undefined when operating over Unix domain sockets, custom IPC transports, or synthetic test injections. Previously, !isIPv6(undefined) evaluated to true, causing undefined.toLowerCase() to throw TypeError: Cannot read properties of undefined (reading 'toLowerCase') and returning an unhandled 500.
• Guarding with if (typeof ip !== 'string') return '' at the top of normalizeIP safely catches undefined, null, or non-string inputs and returns a safe fallback key.
• Verified test coverage in test/ip-normalization.test.js covering helper invocation and end-to-end route injection with an undefined req.ip. All 16 CI checks on Node 20 through 26 pass.