Fix AmiVector::nvec_ for size 0

[dweindl]

• We still need to create an NVector, even if it's empty.
• Don't pass nullptr as sunctx

Issue introduced in #2513.

Closes #2534.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.78%. Comparing base (2784887) to head (62179fb).
Report is 1 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #2535   +/-   ##
========================================
  Coverage    77.77%   77.78%           
========================================
  Files          325      325           
  Lines        21048    21047    -1     
  Branches      1472     1473    +1     
========================================
  Hits         16371    16371           
+ Misses        4650     4649    -1     
  Partials        27       27           

Flag	Coverage Δ
cpp	73.49% <100.00%> (+<0.01%)	⬆️
cpp_python	34.02% <75.00%> (-0.01%)	⬇️
petab	37.03% <50.00%> (-0.02%)	⬇️
python	72.32% <75.00%> (-0.01%)	⬇️

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Files with missing lines	Coverage Δ
include/amici/vector.h	85.36% <100.00%> (+2.43%)	⬆️
src/solver.cpp	77.03% <100.00%> (ø)
src/vector.cpp	86.20% <100.00%> (-0.16%)	⬇️