In gimp_image_set_colormap(), make sure the image's colormap always
has at least one color -- babl palette formats must have at least
one color.

If the function is called with 0 colors, use black.  We still need
to support this case, in particular, since existing XCFs may have
an empty colormap, and since plug-ins can call
gimp-image-set-colormap with 0 colors.