FixedSizeDiffCache use Nothing instead of nothing

[jClugstor]

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

In the construction of FixedSizeDiffCache we create the Dual numbers with tag nothing. This changes it to use Nothing for the tag, which is consistent with zero for Dual numbers.

julia> zero(ForwardDiff.Dual)
Dual{Nothing}(0)

This is related to EnzymeAD/Enzyme.jl#2371.

[jClugstor]

Well it looks like that messed with a lot more than I thought it would...

[ChrisRackauckas]

It looks fine?

[jClugstor]

Oh yeah, looks like the build kite failures are just CUDA failing to initialize, which shouldn't be related to this.

[jClugstor]

@ChrisRackauckas test is in here now