Add a conditional to match `inc_usage_count()`

Otherwise, for empty $used_by values, it will fire off a needless db
query.

includes/class-wc-coupon.php

@@ -271,6 +271,7 @@ class WC_Coupon {
 			$this->usage_count--;
 			update_post_meta( $this->id, 'usage_count', $this->usage_count );
 
+			if ( $used_by ) {
 				/**
 				 * We're doing this the long way because `delete_post_meta( $id, $key, $value )` deletes
 				 * all instances where the key and value match, and we only want to delete one.
@@ -281,6 +282,7 @@ class WC_Coupon {
 				}
 			}
 		}
+	}
 
 	/**
 	 * Returns the error_message string