Allow FiniteDiff propagation for scalar problems

[avik-pal]

Checklist

• [x] Appropriate tests were added
• [x] Any code changes were done in a way that does not break public API
• [x] The new code follows the contributor guidelines, in particular the SciML Style Guide and COLPRAC.

Additional context

Fixes #408

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 86.86%. Comparing base (e295922) to head (0dc75b4).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #409 +/- ## ========================================== + Coverage 86.71% 86.86% +0.15% ========================================== Files 46 46 Lines 2845 2855 +10 ========================================== + Hits 2467 2480 +13 + Misses 378 375 -3 ```

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