Media Manager : fix filtering for empty strings fixes xibosignage/xibo#2859
Media Manager : fix filtering for empty strings
fixes xibosignage/xibo#2859

xibosignage/xibo-cms,dasgarner/xibo-cms,dasgarner/xibo-cms,dasgarner/xibo-cms,xibosignage/xibo-cms,dasgarner/xibo-cms,dasgarner/xibo-cms,dasgarner/xibo-cms,xibosignage/xibo-cms,xibosignage/xibo-cms,xibosignage/xibo-cms,xibosignage/xibo-cms