fix: handle `:var => nothing` provided to `remake` #3396

Job	Run time
DelayDiffEq.jl/Interface/1	19m 21s
DelayDiffEq.jl/Integrators/1	30m 47s
DelayDiffEq.jl/Regression/1	14m 1s
DiffEqBase.jl/Core/1	7m 26s
DiffEqDevTools.jl/Core/1	33m 55s
DiffEqBase.jl/Downstream/1	21m 43s
DiffEqBase.jl/Downstream2/1	22m 32s
DiffEqFlux.jl/DiffEqFlux/1	21m 13s
JumpProcesses.jl/All/1	27m 11s
OrdinaryDiffEq.jl/Interface/1	57m 27s
OrdinaryDiffEq.jl/Integrators/1	16m 54s
OrdinaryDiffEq.jl/Regression/1	30m 0s
StochasticDiffEq.jl/Interface1/1	33m 7s
StochasticDiffEq.jl/Interface2/1	26m 14s
StochasticDiffEq.jl/Interface3/1	23m 57s
StochasticDiffEq.jl/AlgConvergence/1	21m 24s
Sundials.jl/All/1	11m 54s
LinearSolve.jl/All/1	6m 17s
NonlinearSolve.jl/All/1	31m 29s
Optimization.jl/All/1	25m 27s
Integrals.jl/All/1	9m 43s
BoundaryValueDiffEq.jl/All/1	30m 42s
ModelingToolkit.jl/All/1	1h 30m 39s
ModelingToolkitStandardLibrary.jl/Core/1	30m 28s
StochasticDelayDiffEq.jl/All/1	23m 29s
SimpleNonlinearSolve.jl/All/1	5m 45s
SimpleDiffEq.jl/All/1	8m 19s
SciMLSensitivity.jl/Core1/1	46m 18s
SciMLSensitivity.jl/Core2/1	42m 59s
SciMLSensitivity.jl/Core3/1	40m 42s
SciMLSensitivity.jl/Core4/1	38m 47s
SciMLSensitivity.jl/Core5/1	33m 49s
Catalyst.jl/All/1	1h 5m 20s
	15h 49m 19s

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