Fix ProblemExtensions.sciml defaulting to non-None

[dweindl]

Fixes #512.

In some places, problem.extensions.sciml was assumed to be None for non-sciml problems, but it wasn't. In other places it was assumed to always be a SciMLExt instance. I find the former more intuitive and updated the code accordingly.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.75%. Comparing base (6629df6) to head (35ec295).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #513      +/-   ##
==========================================
- Coverage   75.80%   75.75%   -0.06%     
==========================================
  Files          65       65              
  Lines        7357     7357              
  Branches     1323     1323              
==========================================
- Hits         5577     5573       -4     
- Misses       1280     1285       +5     
+ Partials      500      499       -1     

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