REST API: Cast empty meta values to correct scalar types in REST response.

Introducing complex meta value handling in [45807] unintentionally removed value casting for empty scalar meta values.

Props TimothyBlynJacobs, chrisvanpatten, rmccue, kadamwhite.
Fixes #48363.


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git-svn-id: https://core.svn.wordpress.org/trunk@46360 1a063a9b-81f0-0310-95a4-ce76da25c4cd