Fix pullback of double derivatives on Piola type elements (#312)

* Add handling of case where a split returns a form with no arguments (they have all been reduced to zeros).

* Add test case

* Add test and some error handling

* Apply restriction in formsplitter

* Fix docstring

* Fix apply pullback on piola mapped elements for double derivatives

ufl/algorithms/apply_derivatives.py

@@ -30,6 +30,8 @@
  Imag,
  Indexed,
  IndexSum,
+ Jacobian,
+ JacobianDeterminant,
  JacobianInverse,
  ListTensor,
  Product,
@@ -672,7 +674,7 @@ def reference_grad(self, o):
  f = o.ufl_operands[0]
 
  valid_operand = f._ufl_is_in_reference_frame_ or isinstance(
- f, (JacobianInverse, SpatialCoordinate)
+ f, (JacobianInverse, SpatialCoordinate, Jacobian, JacobianDeterminant)
  )
  if not valid_operand:
  raise ValueError("ReferenceGrad can only wrap a reference frame type!")