GETMAPLEGENDIMAGE should have option to omit empty layer groups from being rendered

[jng]

The current GETMAPLEGENDIMAGE operation has a limitation whereby all layer groups in the current map are included in the rendered legend image.

If the map in question has lots of layer groups but the layers contained within are not visible in the current map view/scale, then this results in a rendered image containing lots of empty layer groups, cluttering up the image.

The GETMAPLEGENDIMAGE operation should be enhanced to specify whether empty layer groups should be rendered to the resulting legend image.