Specifically check that child nodes are not strictly equal to `false` so that the value zero (0) gets rendered correctly.

Also add explicit checks for `undefined` and `null` (I’ve encountered `undefined` in my testing with [Kitten](https://codeberg.org/kitten/app). Haven’t hit `null` but it would also make it panic, so better safe than sorry). We could use a lax equality comparison (`==`) here but I prefer being explicit about it (but happy either way).