fix: Invalid return value for commission rate validate method (#2490)

includes/Commission.php

@@ -287,7 +287,7 @@ public function get_earning_by_order( $order, $context = 'seller' ) {
      */
     public function validate_rate( $rate ) {
         if ( '' === $rate || ! is_numeric( $rate ) || $rate < 0 ) {
-            return null;
+            $rate = 0.0;
         }
 
         return (float) $rate;