Automatically prepopulate registered meta keys in the Custom Fields meta box

[psykro]

When the Custom fields meta box is enabled, it does not provide the option to select from any meta fields registered using either register_meta or register_post_meta.

Once the query has been run to fetch any post_meta keys from the table, this checks if that array contains values. If it does not, it fetches any registered post meta keys and uses that array instead.

Trac ticket: https://core.trac.wordpress.org/ticket/62664#ticket