REST API: Return a `WP_Error` if `meta` property is not an array.

Fixes bug where a PHP Warning is currently thrown if a client sends a request where `meta` is not an array value.

Props timmydcrawford, jnylen0, rachelbaker, pento.
Fixes #38989.
Built from https://develop.svn.wordpress.org/trunk@39436


git-svn-id: https://core.svn.wordpress.org/trunk@39376 1a063a9b-81f0-0310-95a4-ce76da25c4cd