[PixelGordo]

Solved, it's always the same, you strugle with something for a long time (not that much, just a couple of days) and inmediately after you ask for help, you finally find the solution. For completeness, I include it here, maybe it'll help somebody in the future.

Short answer: NO, String.IsEmpty doesn't work with variables.

...but there is a simple hack or workaround:

You have to create a dummy text label. It'll be invisible and it'll contain the text of the variable:

xml:


<control type="label" id="6661">

    <description>Dummy label to control the visibility of fanart image</description>

    <label>$VAR[MetaFanart]</label>

    <visible>false</visible>

</control>

Then, you control the visibility of your "real" control checking if the dummy control is empty or not:

xml:
<control type="image">

    <description>Fanart Image</description>

    <include>BackgroundDimensions</include>

    <texture>$VAR[MetaFanart]</texture>

    <visible>!String.IsEmpty(Control.GetLabel(6661))</visible>

</control>