use Zero rather than DoesNotExist for NO_FIELDS

Author: oxinabox

src/differentials/composite.jl

@@ -205,4 +205,4 @@ Constant for the reverse-mode derivative with respect to a structure that has no
 The most notable use for this is for the reverse-mode derivative with respect to the
 function itself, when that function is not a closure.
 """
-const NO_FIELDS = DoesNotExist()
+const NO_FIELDS = Zero()

test/differentials/composite.jl

@@ -155,4 +155,8 @@ end
             @test (@allocated ChainRulesCore.elementwise_add(bk, bk)) <= 48
         end
     end
+
+    @testset "NO_FIELDS" begin
+        @test NO_FIELDS === Zero()
+    end
 end