GEOS-5710 GetLegendGraphic for a layergroup with a coverage invisble at current scale fails

If a layergroup contains a coverage, asking a legend for the group throws an IllegalArgumentException if the coverage is not visible at current scale, because there is no rule applicable for the coverage.
RasterLayerLegendHelper should simply return a null image in this case, letting BufferedImageLegendGraphicBuilder collect legends for the other group layers.