Fix findall with empty MtlArray of Bool (#601)

Author: christiangnrd

src/indexing.jl

@@ -28,7 +28,7 @@ function Base.findall(bools::WrappedMtlArray{Bool})
     indices = MtlVector{Int64, Metal.SharedStorage}(undef, boolslen)
     cumsum!(indices, reshape(bools, boolslen))
 
-    n = indices[end]
+    n = isempty(indices) ? 0 : indices[end]
     ys = similar(bools, I, n)
 
     if n > 0

test/array.jl

@@ -533,6 +533,9 @@ end
     end
 
     # ND
+    let x = rand(Bool, 0, 0)
+      @test findall(x) == Array(findall(MtlArray(x)))
+    end
     let x = rand(Bool, 1000, 1000)
         @test findall(x) == Array(findall(MtlArray(x)))
     end