feat: allow initialization of null integrators

[AayushSabharwal]

Requires

• fix: fix bug in remake_buffer ModelingToolkit.jl#2729
• fix: remake initialization problem during DAE initialization #2209
• feat: solve initialization problem for null solutions/integrators DiffEqBase.jl#1051
• feat: add initializeprob_updatep! to ODEFunction, DAEFunction SciMLBase.jl#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 SciML/ModelingToolkit.jl#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!

[AayushSabharwal]

Superseded by #2314