fix: guard against null Command.Item value (TypeError on 'in' operator)

[guuszz]

Bug

<Command.Item value={null}> throws TypeError: Cannot use 'in' operator to search for 'current' in null instead of rendering. Reported in #404.

Root cause

In useValue (cmdk/src/index.tsx), each part of the value dependencies is inspected with:

if (typeof part === 'object' && 'current' in part) {

When value={null}, part is null. Because typeof null === 'object' (the long-standing JavaScript quirk), the first condition passes and execution reaches 'current' in null, which throws — the in operator requires an object on its right-hand side.

Fix

Add a truthiness guard so null/undefined short-circuit before reaching the in operator:

if (part && typeof part === 'object' && 'current' in part) {

Real refs are truthy objects, so existing behavior is unchanged — only the null/undefined case is now handled gracefully.

Fixes #404