fix(sitemap): don't drop lastmod for an epoch (0) timestamp

[durvesh1992]

Motivation

createSitemapItem decides a route's <lastmod> from route.metadata.lastUpdatedAt (a numeric timestamp, e.g. derived from front matter last_update: {date: ...}). Two checks use truthiness on that number:

// getRouteLastUpdatedAt
if (route.metadata?.lastUpdatedAt) { ... }            // 0 is falsy → skipped
// getRouteLastmod
return lastUpdatedAt ? formatLastmod(...) : null;      // 0 → null

So a legitimate timestamp of 0 (1970-01-01T00:00:00Z) is treated as "no value" and the <lastmod> is silently omitted. The adjacent === null early-return already shows the intent to distinguish absent from present — the truthiness checks defeat that for 0.

Fix

Use != null in both places so 0 is preserved (while still treating null/undefined as absent).

Test plan

pnpm vitest run packages/docusaurus-plugin-sitemap

Added a regression test (lastUpdatedAt: 0 → lastmod: '1970-01-01') that fails before this change and passes after. Full sitemap package: 53 tests pass.

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