Fix coloring with empty matrix as input

Author: blegat

src/interface.jl

@@ -305,7 +305,13 @@ function _coloring(
         vertices_in_order = vertices(ag, order)
         return acyclic_coloring(ag, vertices_in_order, algo.postprocessing)
     end
-    color, tree_set = argmin(maximum ∘ first, color_and_tree_set_by_order)
+    # if `color` is empty, `maximum` will fail but `color_and_tree_set_by_order`
+    # is also one so we can just add a special case for this
+    if length(color_and_tree_set_by_order) == 1
+        color, tree_set = only(color_and_tree_set_by_order)
+    else
+        color, tree_set = argmin(maximum ∘ first, color_and_tree_set_by_order)
+    end
     if speed_setting isa WithResult
         return TreeSetColoringResult(A, ag, color, tree_set, R)
     else

src/result.jl

@@ -82,6 +82,9 @@ Create a color-indexed vector `group` such that `i ∈ group[c]` iff `color[i] =
 Assumes the colors are contiguously numbered from `0` to some `cmax`.
 """
 function group_by_color(::Type{T}, color::AbstractVector) where {T<:Integer}
+    if isempty(color)
+        return SubArray{Int,1,Vector{Int},Tuple{UnitRange{Int}},true}[]
+    end
     cmin, cmax = extrema(color)
     @assert cmin >= 0
     # Compute group sizes and offsets for a joint storage