[N/A] Fix zero-carbon nitrogen transfer

[Alomir]

Summary

• What: Short description of the functional or documentation changes
• Motivation: Why is this change needed?

How was this change tested?

• All current tests pass
• New test added

Related issues

• Fixes #N/A

Checklist

• Related issues are listed above. PRs without an approved, related issue may not get reviewed.
• PR title has the issue number in it ("[#] <concise description of proposed change>")
• Tests added/updated for new features (if applicable)
• Documentation updated (if applicable)
• docs/CHANGELOG.md updated with noteworthy changes
• Code formatted with clang-format (run git clang-format if needed)

[Copilot]

Copilot review overview

🟢 Approval recommended

The focused guards address the zero-division paths and are covered by an appropriate regression test.

Review effort: Balanced
Findings: None

What changed in this PR

Prevents non-finite nitrogen fluxes when zero-carbon pools produce zero transfers.

Changes:

• Guards zero-carbon nitrogen transfer calculations.
• Adds regression coverage for empty carbon pools and nitrogen conservation.
• Documents the fix.

File	Description
src/​sipnet/​nitrogen.c	Safely handles zero carbon fluxes.
tests/​sipnet/​test_modeling/​testNitrogenCycle.c	Tests finite, conserved nitrogen pools.
docs/​CHANGELOG.md	Records the fix.

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[Alomir]

I strongly suspect this is the source of the NaNs that @infotroph was seeing when pools dropped to zero; I believe those started with litter N

[Alomir]

@dlebauer I believe you need to merge this with master?