REST API: Treat any falsy value as `false` in 'rest_allow_anonymous_comments'.

Extend the check in 'rest_allow_anonymous_comments' to accept any falsy value
(previously this was an explicit check for `false`).

One possible failure case is that a plugin developer forgets to include a
return value for some code path in their callback for this filter, leading to a
value of `null` which is currently treated like `true`.

Props joehoyle, jnylen0.

Fixes #39010.

Built from https://develop.svn.wordpress.org/trunk@39487


git-svn-id: https://core.svn.wordpress.org/trunk@39427 1a063a9b-81f0-0310-95a4-ce76da25c4cd