fix: allow (empty) sparse Hessians of linear functions

[gdalle]

Fixes JuliaDiff/SparseMatrixColorings.jl#281

Related: #857

Note that the fix is not necessary yet for unidirectional Jacobians because there is no postprocessing that can lead to zero groups

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.15%. Comparing base (9f50690) to head (60f7a75).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #906   +/-   ##
=======================================
  Coverage   98.15%   98.15%           
=======================================
  Files         134      134           
  Lines        8018     8022    +4     
=======================================
+ Hits         7870     7874    +4     
  Misses        148      148           

Flag	Coverage Δ
DI	98.85% <100.00%> (+<0.01%)	⬆️
DIT	96.39% <ø> (ø)

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[franckgaga]

It solves the issue on my side, good work!