Fix parameter-only gradients

[klamike]

This PR skips the grad/hess structure passes when the graphs evaluate to NaN, effectively marking them as having zero terms. I think this only happens when the constraint consists of only a parameter has no variables.

I'm not too confident in this fix, it might be better to adjust these methods instead:

ExaModels.jl/src/graph.jl

Lines 142 to 144 in bde7733

	@inline (v::ParameterNode{I})(i, x, ::Nothing) where {I<:AbstractNode} = NaN
	@inline (v::ParameterNode{I})(::Any, x, ::Nothing) where {I} = NaN
	@inline (v::ParameterNode{I})(::Identity, x, ::Nothing) where {I<:AbstractNode} = NaN

But I'm not sure how. A related case is constraint(c, ::Int), which ends up calling _simdfunction(Null(), ...) but in that case AdjointNull is fine since it's a leaf node. Please advise 😄