feat: allow initialization of null integrators

[AayushSabharwal]

Requires

• https://github.com/SciML/ModelingToolkit.jl/pull/2729
• https://github.com/SciML/OrdinaryDiffEq.jl/pull/2209
• https://github.com/SciML/DiffEqBase.jl/pull/1051
• https://github.com/SciML/SciMLBase.jl/pull/698

Checklist

• [ ] Appropriate tests were added
• [ ] Any code changes were done in a way that does not break public API
• [ ] All documentation related to code changes were updated
• [ ] The new code follows the contributor guidelines, in particular the SciML Style Guide and COLPRAC.
• [ ] Any new documentation only uses public API

Additional context

Add any other context about the problem here.

[ChrisRackauckas]

What's a case that needs this?

[AayushSabharwal]

This was part of the rework that allows parameters to be unknowns in the initialization system. That bit is on hold for the time being, pending the discussion in https://github.com/SciML/ModelingToolkit.jl/issues/2665

[ChrisRackauckas]

Don't you just need to add the initializationprob call up here? https://github.com/SciML/DiffEqBase.jl/blob/master/src/solve.jl#L678

[AayushSabharwal]

Hmm yeah that does look better. I'll try that, thanks!