Guards building/ceiling/site/wall renderers with `(node.children ?? [])` so a node whose `children` array is missing (legacy/unparsed scene data) no longer crashes the renderer with "Cannot read properties of undefined (reading 'map')" (EDITOR-C0). Matches the existing guard in roof/renderer.tsx and the Array.isArray check in the parametric renderer.
Note: the schema declares `children: z.array(...).default([])`, so this can only be hit by data that bypasses Zod normalization on load. This is a defense-in-depth crash-stopper; the deeper fix is to normalize/parse legacy nodes in migrateNodes (use-scene.ts) so missing arrays are repaired before render — tracked as a follow-up.
