Warn users of shipping rates to test for existence of meta_data correctly.

As per the info in the issue itself, using `isset()` gives incorrect result all the time, so warn developers about the problem.

plugins/woocommerce/includes/class-wc-shipping-rate.php

@@ -65,6 +65,9 @@ class WC_Shipping_Rate {
 	 * @return bool
 	 */
 	public function __isset( $key ) {
+		if ( 'meta_data' === $key ) {
+			wc_doing_it_wrong( __FUNCTION__, __( 'Use `array_key_exists` to check for meta_data on Shipping rates to get the correct result.', 'woocommerce' ), '6.0' );
+		}
 		return isset( $this->data[ $key ] );
 	}