Allow FiniteDiff propagation for scalar problems

[avik-pal]

Checklist

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

Additional context

Fixes #408

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

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

Additional details and impacted files

@@            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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