added null check fro post_meta_time getter

includes/transformer/class-base.php

@@ -121,6 +121,9 @@ abstract class Base { // todo renmae to Base_tranformer
 	 */
 	public function get_post_meta_time( string $key ) {
 		$time = $this->get_post_meta( $key, true ); // todo what if not exists
+		if ( null === $time ) {
+			return null;
+		}
 		$time = \strtotime( $time );
 		return \gmdate( $this->timeformat, $time );
 	}