Extend FormSplitter to handle restrictions

[jorgensd]

When extracting blocks, one can end up with PositiveRestriction(Zero()) in the form when using dS integrals, and extracting a block with no contribution.
MWE:

   domain = ufl.Mesh(FiniteElement("Lagrange", cell, 1, (d,), ufl.identity_pullback, ufl.H1))
    el_u = FiniteElement("Lagrange", cell, 2, (d, ), ufl.identity_pullback, ufl.H1)
    el_p = FiniteElement(
        "Lagrange", cell, 1, (), ufl.identity_pullback, ufl.H1)
    V = ufl.FunctionSpace(domain, el_u)
    Q = ufl.FunctionSpace(domain, el_p)
    W = ufl.MixedFunctionSpace(V, Q)
    u, p = ufl.TrialFunctions(W)
    v, q = ufl.TestFunctions(W) 
    a = ufl.inner(ufl.grad(u), ufl.grad(v))*ufl.dx + ufl.div(u)*q*ufl.dx + ufl.div(v)*p*ufl.dx
    a += ufl.inner(u("+"),v("+")) * ufl.dS
    a_blocks = ufl.extract_blocks(a)
    assert a_blocks[1][1] is None

does not return a zero form.

This PR extends FormSplitter such that
split(Restriction(expr))
now returns
Zero if split(expr)=Zero