Fix null mounts projection in proven unallocated Volume classification

[chen21019]

Root cause

Packaged QA v1.6.506 has the exact inactive local Volume in the Store with a
complete empty pool Collection and scoped mount cache. Its API-expanded
mounts: null projection is preserved by the resource setter but rejected by
the shared classifier, so the native table hides the resource.

Minimal repair

• Accept null only for the optional expanded mounts projection, never as proof
  of absent allocation and never convert it to a manufactured empty array.
• Retain complete pool relationship, scoped mount-cache, exact-volume mount,
  project, fingerprint, nonempty raw ID and backend authorization guards.
• Require native/Ember arrays rather than arbitrary objects with length: 0.
• Add actual Store/Volume regression coverage and align the numeric 1.6.170
  release metadata and lock baseline without changing the dependency graph.

Focused Chrome 153: 13/13 pass, zero failures/skips/todo. Independent scoped
source review: no blocker. Official immutable archive validation and packaged
QA lifecycle acceptance remain separate; historical HOLDs and the full matrix
are not promoted. No API/Auth/Engine, HAProxy or company deployment changes.