Empty form does not have arguments/arity

Observed in the context of https://github.com/FEniCS/ufl/pull/392 and discussions with David.

An empty form, stemming for instance from differentiation, should contain information with respect to what spaces it "belongs" to, i.e. an empty form of a rank-1 form should be different than an empty rank-2 form.

Potentially this means that one should use `ZeroBaseForm` to replace zeros.