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.

Built from https://develop.svn.wordpress.org/trunk@39594


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

wp-includes/rest-api/class-wp-rest-request.php

@@ -669,7 +669,12 @@ class WP_REST_Request implements ArrayAccess {
 			return true;
 		}
 
-		$params = json_decode( $this->get_body(), true );
+		$body = $this->get_body();
+		if ( empty( $body ) ) {
+			return true;
+		}
+
+		$params = json_decode( $body, true );
 
 		/*
 		 * Check for a parsing error.