The `register_meta()` API now officially supports specifying a default metadata value. When `get_metadata()` is called for a meta key that does not yet exist for the object, this default value will be returned instead of an empty string.
A new function is introduced `get_metadata_raw` to retrieve the raw metadata value from the database, without applying the registered default.
Props spacedmonkey, flixos90, rmccue, kadamwhite, mnelson4, johnbillion, chrisvanpatten, TimothyBlynJacobs.
Fixes#43941.
git-svn-id: https://develop.svn.wordpress.org/trunk@48402 602fd350-edb4-49c9-b593-d223f7449a82
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