REST API: Add tests for empty or "no-op" updates.

The API should allow updates that don't actually change anything.  This allows
clients to, for example, accidentally send the same request twice without
encountering unexpected errors.  This currently works for posts, terms, and
users, so this commit adds test cases accordingly.

See #38700 for issues preventing this from working for comments.

Fixes #38975.

Built from https://develop.svn.wordpress.org/trunk@39371


git-svn-id: https://core.svn.wordpress.org/trunk@39311 1a063a9b-81f0-0310-95a4-ce76da25c4cd