Update WC_Queue_Interface::get_next()

To more closely align return value with other date APIs in WC, namely
WC_Data. Includes changing return value to:

1. a WC_DateTime instead of a timestamp when a scheduled occurrence is found
2. null instead of false, similar to the default of WC_Data::set_date_prop()

includes/interfaces/class-wc-queue-interface.php

@@ -86,10 +86,14 @@ interface WC_Queue_Interface {
 	public function cancel( $hook, $args = array(), $group = '' );
 
 	/**
+	 * Get the date and time for the next scheduled occurence of an action with a given hook
+	 * (an optionally that matches certain args and group), if any.
+	 *
 	 * @param string $hook
 	 * @param array $args
 	 * @param string $group
 	 * @return int|bool The timestamp for the next occurrence, or false if nothing was found
+	 * @return WC_DateTime|null The date and time for the next occurrence, or null if there is no pending, scheduled action for the given hook
 	 */
 	public function get_next( $hook, $args = null, $group = '' );
 

includes/queue/class-wc-action-queue.php

@@ -101,14 +101,24 @@ class WC_Action_Queue implements WC_Queue_Interface {
 	}
 
 	/**
+	 * Get the date and time for the next scheduled occurence of an action with a given hook
+	 * (an optionally that matches certain args and group), if any.
+	 *
 	 * @param string $hook
 	 * @param array $args
 	 * @param string $group
-	 *
 	 * @return int|bool The timestamp for the next occurrence, or false if nothing was found
+	 * @return WC_DateTime|null The date and time for the next occurrence, or null if there is no pending, scheduled action for the given hook
 	 */
-	public function get_next( $hook, $args = NULL, $group = '' ) {
-		return wc_next_scheduled_action( $hook, $args, $group );
+	public function get_next( $hook, $args = null, $group = '' ) {
+
+		$next_timestamp = wc_next_scheduled_action( $hook, $args, $group );
+
+		if ( $next_timestamp ) {
+			return wc_string_to_datetime( $next_timestamp );
+		}
+
+		return null;
 	}
 
 	/**