[Utilities] ignore attributes in UniversalFallback if set to nothing

[odow]

x-ref jump-dev/Gurobi.jl#584

We don't have a way to remove attributes once they've been set. We've mostly settled on nothing as the resetting to default.

For example

MathOptInterface.jl/src/attributes.jl

Lines 329 to 331 in 278fba5

	If the attribute `attr` is set for the variable `v` in the model `model`, return
	its value, return `nothing` otherwise. If the attribute `attr` is not supported
	by `model` then an error should be thrown instead of returning `nothing`.

MathOptInterface.jl/src/attributes.jl

Lines 848 to 850 in 278fba5

	An optimizer attribute for setting a time limit (in seconds) for an
	optimization. When `set` to `nothing`, it deactivates the solver time limit. The
	default value is `nothing`.

MathOptInterface.jl/src/attributes.jl

Lines 861 to 863 in 278fba5

	The provided limit must be a `Union{Real,Nothing}`.
	When `set` to `nothing`, the limit reverts to the solver's default.

[odow]

This needs some tests

[odow]

https://github.com/jump-dev/MathOptInterface.jl/actions/runs/11655223284

MiniZinc failure is unrelated and jump-dev/MiniZinc.jl#89

[odow]

solver-tests.yml shows no issues.

[blegat]

Another approach would be to simply delete it from the dictionary when it's set to nothing