Fix PetabSimulator._aggregate_s2llh

[dweindl]

Only try to aggregate ReturnData.FIM if we can expect it to be computed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.58%. Comparing base (27184ac) to head (3932c9e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3125      +/-   ##
==========================================
- Coverage   78.27%   77.58%   -0.70%     
==========================================
  Files         315      315              
  Lines       20482    20482              
  Branches     1484     1484              
==========================================
- Hits        16033    15891     -142     
- Misses       4441     4583     +142     
  Partials        8        8              

Flag	Coverage Δ
cpp	71.71% <0.00%> (-0.03%)	⬇️
cpp_python	37.85% <0.00%> (ø)
petab	46.56% <100.00%> (ø)
petab_sciml	13.93% <0.00%> (ø)
python	70.36% <100.00%> (-0.03%)	⬇️
sbmlsuite-jax	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
python/sdist/amici/sim/sundials/petab/_v2.py	93.20% <100.00%> (ø)

... and 6 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.