Model import: Don't create spl.h and sspl.h for models without splines

[dweindl]

Closes #2087

[codecov]

Codecov Report

Merging #2088 (4156d63) into develop (c3595c4) will increase coverage by 0.00%.
The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #2088   +/-   ##
========================================
  Coverage    76.10%   76.11%           
========================================
  Files           80       80           
  Lines        14838    14840    +2     
========================================
+ Hits         11293    11296    +3     
+ Misses        3545     3544    -1     

Flag	Coverage Δ
cpp	73.56% <ø> (+0.01%)	⬆️
petab	49.46% <100.00%> (+0.01%)	⬆️
python	76.87% <100.00%> (+<0.01%)	⬆️
sbmlsuite	∅ <ø> (∅)

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Impacted Files	Coverage Δ
python/sdist/amici/de_export.py	91.60% <100.00%> (+0.01%)	⬆️

... and 1 file with indirect coverage changes

[FFroehlich]

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