Ensure Snapshot.Disks marshals as [] instead of null

[LarsLaskowski]

📖 Description

docs/API.md documents that fields may briefly read as "empty arrays" right after process start, before the first collection tick completes. In practice, Snapshot.Disks []Disk has no omitempty (unlike Network), and its Go zero value (nil) marshals to JSON null, not []. Strict JSON consumers iterating disks would trip on null. This also applied after any tick where disk collection failed or found no eligible mountpoints (DiskCollector.Collect used var disks []Disk, which stays nil when nothing is appended).

Fix: keep Snapshot.Disks a non-nil (possibly empty) slice everywhere:

• internal/collector/disk.go: DiskCollector.Collect now allocates with make([]Disk, 0, len(order)) instead of a nil var disks []Disk.
• internal/collector/collector.go: the Collector's initial latest snapshot now starts with Disks: []Disk{} (covers the window before the first fast tick), and fastTick normalizes a nil result from DiskCollector.Collect (e.g. on a read error) before storing it.

This is a docs-consistency fix, not a JSON shape change — the field itself (disks) is unchanged, only its zero-state value goes from null to [], which is what docs/API.md already promised.

🎫 Issues

Closes #70

👩‍💻 Reviewer Notes

Changes are confined to internal/collector. No API surface, config, or packaging changes.

📑 Test Plan

• Added TestCollector_Disks_NeverMarshalsAsNull in internal/collector/collector_test.go, which JSON-marshals a Snapshot both before the first fastTick and after, asserting "disks" is never "null".
• Added TestDiskCollector_Collect_NoMountsReturnsEmptyNotNilSlice in internal/collector/disk_test.go covering the case where no mountpoint qualifies (all excluded pseudo filesystems).
• Extended the existing TestDiskCollector_Collect_SkipsFailingStatfs to assert the returned slice is non-nil.
• go build ./..., go vet ./..., go test ./... -race -cover, and golangci-lint run all pass locally.

✅ Checklist

General

• I have added/updated tests for my changes (go test ./... -race -cover passes locally).
• go vet ./... and golangci-lint run are clean.
• I have tested my changes.
• I have read the CONTRIBUTING documentation and followed the project's code style guidelines.
• I have updated ARCHITECTURE.md if this changes a documented design decision. (not applicable — no design change)

REST API / configuration / packaging

• I have updated docs/API.md to reflect a REST API change. (not needed — the doc already describes this exact behavior; the code now matches it)
• No breaking change to /api/v1/... response shapes, or a new API version (/api/v2/...) was introduced instead.
• I have updated README.md / packaging/pimonitor.example.yaml — (not applicable)
• I have updated packaging/install.sh or the systemd units — (not applicable)

⏭ Next Steps

None.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

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