Fix undefined array key warning in Scheduler::async_batch (#2497)

Author: obenland

.github/changelog/2497-from-description

@@ -0,0 +1,4 @@
+Significance: patch
+Type: fixed
+
+Undefined array key warning in Scheduler::async_batch when called without arguments.

includes/class-scheduler.php

@@ -472,7 +472,7 @@ public static function async_batch() {
 
 		self::lock( $key );
 
-		if ( \is_callable( $args[0] ) ) {
+		if ( \is_callable( $args[0] ?? null ) ) {
 			$callback = \array_shift( $args ); // Remove $callback from arguments.
 		}
 		$next = \call_user_func_array( $callback, $args );