Ticket #43691: 43691-2.diff

src/wp-includes/rest-api/class-wp-rest-server.php

@@ -401 +401 @@
                          */
                         $result = apply_filters( 'rest_pre_echo_response', $result, $this, $request );
 
+                        // The 204 response shouldn't have a body.
+                        if ( $code === 204 ) {
+                                return null;
+                        }
+
+                        if ( $result === null ) {
+                                _doing_it_wrong( __FUNCTION__, __( 'The response doesn\'t contain a body, which was expected to be there.' ), '5.3' );
+                        }
+
                         $result = wp_json_encode( $result );
 
                         $json_error_message = $this->get_json_last_error();