REST API: Do not error on empty JSON body

It's fairly common for clients to send `Content-Type: application/json` with an
empty body.  While technically not valid JSON, we've historically supported
this behaviour, so it shouldn't cause an error.

Props JPry.
Fixes #39150.

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git-svn-id: https://core.svn.wordpress.org/trunk@39534 1a063a9b-81f0-0310-95a4-ce76da25c4cd