[Utilities] get_fallback assumes Float64

[odow]

See various hard-coded 0.0:

MathOptInterface.jl/src/Utilities/results.jl

Lines 367 to 380 in 145e7bc

	function variable_dual(
	model::MOI.ModelLike,
	attr::MOI.ConstraintDual,
	ci::MOI.ConstraintIndex,
	vi::MOI.VariableIndex,
	F::Type{<:MOI.AbstractFunction},
	S::Type{<:MOI.AbstractSet},
	)
	dual = 0.0
	for constraint_index in MOI.get(model, MOI.ListOfConstraintIndices{F,S}())
	dual += variable_dual(model, attr, vi, constraint_index)
	end
	return dual
	end

Found by #2411