Fieldsplit: replace empty Forms with ZeroBaseForm

[pbrubeck]

Description

This PR addresses the consequences of FEniCS/ufl#336, which introduces more agressive simplifications that cause empty forms to be returned when extracting a subblock that is supposed to be zero (e.g. the pressure block in Stokes).

Here empty Jacobians are replaced with ZeroBaseForm, as we require two Arguments in order for the adjoint to make sense.

The simplifications should also speed up code generation and even reduce flop counts in some cases.

Description update after merge:

This PR also includes updates from #3922. That PR changes the default behaviour of assemble when dealing with boundary conditions, and may also throw errors when trying to restore the old defaults. The description of that PR is:

Description

This PR enables solve(a == assemble(L), u, bcs, restrict={False|True}) without requiring users to set the boundary nodes to zero when they provide a Cofunction RHS. Here we also solved a few other issues:

1. V and V.dual() do no longer compare to the same thing.
2. assemble(form, bcs, zero_bc_nodes=True) is the new default, and for the False case we throw an error because we will try to apply primal inhomogenous data on a dual Cofunction.

Depends on firedrakeproject/ufl#53

Fixes #3203, #3498, and #3130

Supersedes #3214, #3754

[github-actions]

	Tests	Passed ✅	Skipped ⏭️	Failed ❌
Firedrake real	8155 ran	7480 passed	675 skipped	0 failed

[github-actions]

	Tests	Passed ✅	Skipped ⏭️	Failed ❌
Firedrake complex	8149 ran	6675 passed	1474 skipped	0 failed

done