Make the A_form and b_form arguments to SystemAssembler optional

At the moment if you just want to assemble a matrix with symmetrically imposed boundary conditions you have to create a dummy b_form:

u = TrialFunction(V)
v = TestFunction(V)
dummy = inner(Constant((1, 0)), v)*dx

# Assemble matrix for LHS
a = inner(curl(u), curl(v))*dx
asm = SystemAssembler(a, dummy, bcs)
A = PETScMatrix(); asm.assemble(A)

This would be much cleaner if the form arguments to SystemAssembler were optional.

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