Avoid sympification in is-None checks

[dweindl]

Don't sympify if we just need to test some expression to be (not) None. Saves some time for expensive-to-sympify expressions like big piecewise constructs for splines.

Requires #2946.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.79%. Comparing base (1b88ab4) to head (f6b7d1d).
⚠️ Report is 134 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2945      +/-   ##
==========================================
- Coverage   79.43%   78.79%   -0.65%     
==========================================
  Files         362      362              
  Lines       23183    23183              
  Branches     1588     1588              
==========================================
- Hits        18415    18266     -149     
- Misses       4756     4905     +149     
  Partials       12       12              

Flag	Coverage Δ
cpp	75.51% <100.00%> (-0.07%)	⬇️
cpp_python	32.93% <100.00%> (ø)
petab	38.49% <100.00%> (ø)
python	73.57% <100.00%> (-0.07%)	⬇️
sbmlsuite-jax	?

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Files with missing lines	Coverage Δ
python/sdist/amici/sbml_import.py	80.02% <100.00%> (-12.07%)	⬇️

... and 7 files with indirect coverage changes

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