fix(string): preserve falsy variable values in template()

[maxtaran2010]

Summary

The template() function uses vars[key] || fallback when doing named substitution, which causes it to fall through to the fallback value whenever a variable is explicitly set to a falsy value — 0, false, or "".

Buggy behavior:

template('{count}', { count: 0 }, 'N/A')    // returns 'N/A', expected '0'
template('{active}', { active: false }, 'unknown') // returns 'unknown', expected 'false'
template('{name}', { name: '' }, 'anonymous') // returns 'anonymous', expected ''

Fix

Replace vars[key] || fallback with key in vars ? vars[key] : fallback so only keys that are actually absent from vars trigger the fallback, not keys that are present but hold a falsy value.

- return str.replace(/\{(\w+)\}/g, (_, key) => vars[key] || ((typeof fallback === 'function' ? fallback(key) : fallback) ?? key))
+ return str.replace(/\{(\w+)\}/g, (_, key) => key in vars ? vars[key] : ((typeof fallback === 'function' ? fallback(key) : fallback) ?? key))

Test plan

• Added test cases for 0, false, and "" values in named template mode
• All existing tests continue to pass (pnpm test — 39 tests passing)

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[antfu]

Instead of in, let's use "hasOwn" to be safer?