Shortcut findfirst for empty arrays.

[tam724]

On empty arrays, findfirst failed with a BoundsError. We can catch this case with isempty and return nothing, which matches the behaviour for non-gpu arrays.

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diff --git a/test/testsuite/indexing.jl b/test/testsuite/indexing.jl
index d8943de..db71049 100644
--- a/test/testsuite/indexing.jl
+++ b/test/testsuite/indexing.jl
@@ -173,7 +173,7 @@ end
 
         # emtpy
         @test compare(findfirst, AT, Bool[])
-        @test compare(x->findfirst(>(0.5f0), x), AT, Float32[])
+        @test compare(x -> findfirst(>(0.5f0), x), AT, Float32[])
     end
 
     @testset "findmax & findmin" begin