fix(mass_model_updater): never skip a deferred tracked field as "unchanged" [sc-13375]

[Maffooch]

Problem

mass_model_updater(..., skip_unchanged=True) (added in #15046) decides whether to write each row by snapshotting its tracked fields before calling function, reading model.__dict__.get(field) to avoid triggering a deferred-field query.

But when a tracked field is deferred — the caller's queryset used .only()/.defer() and omitted it — __dict__ has no entry, so the snapshot reads None. If function then recomputes that field to None, the comparison None == None marks the row "unchanged" and the write is silently dropped, leaving the stale persisted value in the DB.

This bit DefectDojo Pro's risk-mode SLA recalculation: its queryset deferred sla_expiration_date while updating it, so clearing a finding's SLA to None (enforcement disabled) never persisted — the finding kept its old SLA. (Companion fix in dojo-pro adds the field to its .only(...); this PR removes the footgun at the source.)

Fix

Guard the no-op check with model.get_deferred_fields(): if any tracked field is deferred, skip the comparison and always write that row. No extra query; non-deferred rows keep the fast skip-unchanged path.

Tests

Adds two regression tests to unittests/test_mass_model_updater.py:

• test_writes_deferred_field_recomputed_to_none — a deferred field recomputed to None must issue an UPDATE and persist (fails without this fix: 0 UPDATEs).
• test_writes_deferred_field_recomputed_to_value — deferred field updated to a value persists.

Full TestMassModelUpdater suite (11 tests) passes; reverting the guard makes the None test fail with 0 not greater than 0.

Scope

skip_unchanged exists only on dev (not in 3.0.200 / bugfix / master), so released versions are unaffected.

[mtesauro]

Approved

[Maffooch]

Closing in favor or #15114