When `json_encode()` returns a JSON string containing `'null'` in PHP 5.4 or earlier, `wp_json_encode()` will now sanity check the data, as older versions of PHP failed to encode non UTF-8 characters correctly, instead returning `'null'`.

Fixes #30471.

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git-svn-id: http://core.svn.wordpress.org/trunk@30550 1a063a9b-81f0-0310-95a4-ce76da25c4cd