Fix crash for models without state variables

[dweindl]

Previously, nx_solver=0 would have in a crash when calling amici::SUNMatrixWrapper::capacity on SUNMatrixWrapper of 0-sized sparse matrix.

Unrelated: Replace some unnecessary std::vector::at calls.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.91%. Comparing base (9ea6aef) to head (296c776).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #2703   +/-   ##
========================================
  Coverage    77.91%   77.91%           
========================================
  Files          326      326           
  Lines        23004    23004           
  Branches      1479     1479           
========================================
  Hits         17924    17924           
  Misses        5069     5069           
  Partials        11       11           

Flag	Coverage Δ
cpp	74.76% <100.00%> (ø)
cpp_python	33.57% <100.00%> (ø)
petab	39.41% <83.33%> (-0.01%)	⬇️
python	72.95% <83.33%> (-0.01%)	⬇️

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

Files with missing lines	Coverage Δ
src/model.cpp	85.45% <100.00%> (ø)

🚀 New features to boost your workflow:

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