There's no differentiation between parameters background_color and
background_image in sessionstart.hconf being empty or non-existent. In both
cases, no colored background and/or no image is shown. The expected behavior
non-existent --> hard-coded default value, same as out-of-the-box
empty --> inactivated feature
This would probably require either reworking tl-config/hivetool to reflect existing but
empty parameters, or having tl-startup-bg read hiveconf directly.