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.
Merges [39487] to the 4.7 branch.
Fixes#39010.
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