HOTFIX: handle getindex adjoint for scalar u

[DhairyaLGandhi]

There are some cases where .u becomes scalar. Could be when u0 is a float or save_idxs is a Integer for example. This shows up in SciMLSensitivity https://github.com/SciML/SciMLSensitivity.jl/actions/runs/14307459224/job/40094466372?pr=1168#step:6:1036

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.