Allow conditional tooltip rendering

[andrew-vos]

Would be good to have tooltip rendering conditional? I have it set up like that:

      content:
        type: panel
        layout: canvas
        alignment: { h: center }
        elements:
          - type: switch
            switch_on: ~ui_ingame_heroes_active_hero_visible(@id)
            cases:
              true:
                type: image
                visible: 
                sprite: @small_icon
                tooltip: @@  ~ui_ingame_heroes_active_hero_visible(@id) then @description else undefined
                tooltip_placement: left_top
                scale: 0.5
                size: { h: stretch, w: stretch }

                animate:
                  on_visible:
                    xoffset:
                        type: curve
                        curve: curveCubic // this built in curve smoothly goes from 0 to 1 halfway through, and then smoothly back to 0
                        from: 800
                        to: 0
                        duration: 600
                  on_unloading:
                    xoffset:
                        type: curve
                        curve: curveCubic // this built in curve smoothly goes from 0 to 1 halfway through, and then smoothly back to 0
                        from: 0
                        to: 800
                        duration: 600

but even when it's undefined there's a small little empty square rendered