save work on trivial coloring

src/dualtvdd_surrogate.jl

@@ -96,6 +96,7 @@ function step!(ctx::DualTVDDSurrogateState)
     cids = chessboard_coloring(size(ctx.subax))
 
     for (color, ids) in enumerate(cids)
+        isempty(ids) && continue
         p_current = extend(zeros(eltype(ctx.p), size(ctx.p)),
             StaticKernels.ExtensionNothing())
         for i in ids