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Fix ZeroBaseForm holes in derivative expansion and action (#494)

* Fix ZeroBaseForm holes in derivative expansion and action

GateauxDerivativeRuleset had no rule for ZeroBaseForm itself, so
differentiating an already-zero BaseForm (e.g. a Hessian of a Matrix)
raised "Rule not set for ZeroBaseForm" in expand_derivatives.

map_integrands' FormSum branch collapsed to a bare `0` via `sum([])`
when every component vanished, silently dropping the argument shape;
apply_derivatives had a matching fallback that rebuilt an
argument-less Form([]) instead of a properly shaped ZeroBaseForm.

action() only expanded derivatives when the top-level object was
already a Form/BaseFormOperator, so a CoefficientDerivative reachable
only through a FormSum (e.g. from an affine residual F - b) survived
unexpanded inside the resulting lazy Action. Widened the check to
match adjoint()'s existing BaseForm handling.

Finally, ZeroBaseForm inherited BaseForm.empty(), which always
returns False -- give it its own empty() returning True.

* Silence mypy for the new ZeroBaseForm derivative rule

Matches the existing # type: ignore on the neighboring Matrix rule,
whose signature is likewise incompatible with the Expr-typed base
process() the singledispatchmethod registry expects.

* Coerce ZeroBaseForm arguments to a tuple

ZeroBaseForm stored its arguments as-is, so passing a list (e.g. the
output of extract_arguments) made the object unhashable and broke
anything relying on __hash__, such as expression traversal.

* Handle BaseForm in extract_domains

extract_domains only special-cased Form and Integral, falling back to
walking expr as an Expr tree of Arguments/Coefficients for anything
else. That crashes for a BaseForm with no such tree, e.g. a bare
ZeroBaseForm (no _ufl_is_terminal_) or a FormSum mixing a Form with a
Cofunction.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

* Fix ZeroBaseForm._ufl_expr_reconstruct_

BaseForm's default, type(self)(*operands), unpacks... (continued)

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