The deprecated meta data notice fix

I believe the post id needs to exists in the wp_posts table for post meta to really matter, no point in giving a notice for a post meta where the post doesn't exist.

Please review code first, I'm not 100% sure if the curly brackets should be used like that in the ````join```` there. First time using curly brackets like that ;/

On my side, no more notice :+1: