fix latexify of the empty tree

Author: ranocha

Project.toml

@@ -1,7 +1,7 @@
 name = "RootedTrees"
 uuid = "47965b36-3f3e-11e9-0dcf-4570dfd42a8c"
 authors = ["Hendrik Ranocha <mail@ranocha.de> and contributors"]
-version = "2.6.2-pre"
+version = "2.6.2"
 
 [deps]
 LinearAlgebra = "37e2e46d-f89d-539d-b4ee-838fcccc9c8e"

src/RootedTrees.jl

@@ -1448,6 +1448,9 @@ julia> rootedtree([1, 2, 3, 3, 2]) |> RootedTrees.latexify |> println
 ```
 """
 function latexify(t::RootedTree)
+  if isempty(t)
+    return "\\varnothing"
+  end
   list_representation = butcher_representation(t, false)
   "\\rootedtree" * replace(list_representation, "τ" => "[]")
 end

test/runtests.jl

@@ -99,6 +99,7 @@ end
   @test RootedTrees.latexify(t1) == latex_string
   @test latexify(t1) == "\$" * latex_string * "\$"
   @test isempty(RootedTrees.subtrees(t1))
+  @test RootedTrees.latexify(empty(t1)) == "\\varnothing"
 
   @inferred order(t1)
   @inferred σ(t1)