Handle case in EnzymeTestUtils of reduction over empties (#3131)

* Handle case in EnzymeTestUtils of reduction over empties

* Use zeros for type stability

* Add test for jvp

* use similar not zeros

---------

Co-authored-by: Katharine Hyatt <katharine.s.hyatt@gmail.com>

Author: kshyatt

lib/EnzymeTestUtils/src/finite_difference_calls.jl

@@ -71,7 +71,7 @@ function j′vp(fdm, f_vec, ȳ, x)
         end
     end
   end
-  mat = transpose(reduce(hcat, ẏs))
+  mat = isempty(ẏs) ? similar(x, 0) : transpose(reduce(hcat, ẏs))
   result = zero(x)
   for i in 1:length(ȳ)
     tp = @inbounds ȳ[i] 

lib/EnzymeTestUtils/test/runtests.jl

@@ -12,4 +12,5 @@ Random.seed!(0)
     include("generate_tangent.jl")
     include("test_forward.jl")
     include("test_reverse.jl")
+    include("test_fd.jl")
 end

lib/EnzymeTestUtils/test/test_fd.jl

@@ -0,0 +1,10 @@
+using Test
+using EnzymeTestUtils
+using EnzymeTestUtils: j′vp
+using FiniteDifferences
+
+@testset "j′vp with empty inputs" begin
+    # we can make f_vec here identity since it cannot be
+    # reached if x is itself empty
+    @test isempty(j′vp(FiniteDifferences.central_fdm(5, 1), identity, Float32[], Float32[]))
+end