REST API: Allow schema sanitization_callback to be set to null to bypass...

REST API: Allow schema sanitization_callback to be set to null to bypass fallback sanitization functions.

The logic in WP_REST_Request->sanitize_params() added in [39091] did not account for `null` or `false` being the sanitization_callback preventing overriding `rest_parse_request_arg()`. This fixes that oversight, allowing the built in sanitization function to be bypassed. See #38593.

Props kkoppenhaver, rachelbaker, jnylen0.
Fixes #39042.

Built from https://develop.svn.wordpress.org/trunk@39563


git-svn-id: https://core.svn.wordpress.org/trunk@39503 1a063a9b-81f0-0310-95a4-ce76da25c4cd