Requirements PATCH cannot clear nullable fields (None-skip idiom)

[CST-100]

Found during the devel integration review (PR #15/#25).

PATCH /api/requirements/{id} skips None values, so rationale/category/verification_method — and notably tbr_owner_id/tbr_due, which gate baselining — can never be cleared once set. The detail page sends explicit null when a field is emptied and silently no-ops (UI shows cleared until reload). A wrong TBR owner is only fixable by toggling tbr off and on.

Same root idiom was fixed for procedure-step caution/required_role during integration (use model_fields_set to distinguish absent from explicit null) — apply the same pattern here, field-by-field where clearing is legitimate.

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