fix: error when interpolating derivatives of observed variables

[AayushSabharwal]

Close https://github.com/SciML/ModelingToolkit.jl/issues/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[bot]]

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 :warning:

Additional details and impacted files

```diff @@ 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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