ZeroBaseForm rather than Zero

ufl/algorithms/apply_derivatives.py

@@ -1314,7 +1314,7 @@ def external_operator(self, N, *dfs):
                     *N.ufl_operands, derivatives=derivatives, argument_slots=new_args
                 )
             elif df == 0:
-                extop = Zero(N.ufl_shape)
+                extop = ZeroBaseForm(N.arguments())
             else:
                 raise NotImplementedError(
                     "Frechet derivative of external operators need to be provided!"
@@ -1401,8 +1401,6 @@ def base_form_operator_derivative(self, o, f, dummy_w, dummy_v, dummy_cd):
             return ZeroBaseForm(arguments)
         # We need to go through the dag first to record the pending operations
         mapped_expr = map_expr_dag(rules, f, vcache=self.vcaches[key], rcache=self.rcaches[key])
-        if isinstance(mapped_expr, Zero):
-            mapped_expr = ZeroBaseForm(f.arguments())
         mapped_f = rules.coefficient(f)
         if mapped_f != 0:
             # If dN/dN needs to return an Argument in N space