fix: error when interpolating derivatives of observed variables

[AayushSabharwal]

Close SciML/ModelingToolkit.jl#2697

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.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 10 lines in your changes are missing coverage. Please review.

Project coverage is 4.81%. Comparing base (a0fab7a) to head (5860b14).

Files	Patch %	Lines
src/solutions/ode_solutions.jl	0.00%	10 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #688       +/-   ##
==========================================
- Coverage   31.79%   4.81%   -26.99%     
==========================================
  Files          55      55               
  Lines        4535    4506       -29     
==========================================
- Hits         1442     217     -1225     
- Misses       3093    4289     +1196     

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