Remove case in frule for purely AbstractZero tangent

Author: oxinabox

src/extra_rules.jl

@@ -173,7 +173,7 @@ end
 
 function ChainRules.frule((_, ∂x), ::Type{SArray{S, T, N, L}}, x::NTuple{L,T}) where {S, T, N, L}
     #TODO: we really shouldn't actually see the isa(∂x, AbstractZero) case since the frule should be called then
-    Δx = isa(∂x, AbstractZero) ? ∂x : SArray{S, T, N, L}(ChainRulesCore.backing(∂x))
+    Δx = SArray{S, T, N, L}(ChainRulesCore.backing(∂x))
     SArray{S, T, N, L}(x), Δx
 end