fix: handle empty row or column colors in mixed mode sparse Jacobian

[gdalle]

Fixes #857

Might need additional tests depending on the coverage

[codecov]

Codecov Report

❌ Patch coverage is 90.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 97.22%. Comparing base (fca2ad3) to head (1f2cc3a).

Files with missing lines	Patch %	Lines
...nterfaceSparseMatrixColoringsExt/jacobian_mixed.jl	90.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #864      +/-   ##
==========================================
- Coverage   98.10%   97.22%   -0.89%     
==========================================
  Files         134      131       -3     
  Lines        7984     7970      -14     
==========================================
- Hits         7833     7749      -84     
- Misses        151      221      +70     

Flag	Coverage Δ
DI	97.68% <90.00%> (-1.11%)	⬇️
DIT	96.07% <ø> (-0.33%)	⬇️

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