[Firmware] OTA check can proceed with a bogus master MAC when no Hosts master exists

[AustinChangLinksys]

Summary

Follow-up from PR #1068 review (@PeterJhongLinksys). Non-blocking / low impact.

FirmwareUpdateNotifier.buildOtaCheckParams (firmware_update_notifier.dart:92) dropped the legacy if (masterNode == null) return null guard during the MeshNetwork migration.

devicesData.master is now always non-null — the builder synthesizes a MasterNode(deviceId: 'GATEWAY', ...) when no master/systemInfo is available. So in a degenerate state (no Hosts master resolved) the OTA check no longer short-circuits; instead it proceeds with:

• macAddress: _formatMacAddress('GATEWAY') → a bogus MAC (GATEWAY)
• empty installedVersion / modelNumber

final master = devicesData.master;      // synthetic 'GATEWAY' fallback, never null
...
return FirmwareOtaCheckParams(
  macAddress: _formatMacAddress(master.deviceId),   // 'GATEWAY' → 'GATEWAY'
  installedVersion: master.softwareVersion,          // '' in degenerate case
  modelNumber: master.model,                         // '' in degenerate case
  ...
);

Impact

Low — real routers always have a Hosts master, so this only manifests in a degenerate/transient state. But an OTA check firing with a bogus MAC + empty version/model is worse than being skipped.

Suggested fix

Re-introduce a guard: return null when the master is the synthetic gateway (master.deviceId == 'GATEWAY') or has no hostsDeviceId / empty model, mirroring the legacy skip behavior.

Refs

• Split out of PR fix(devices): MeshNetwork architecture + fix #1043 #1044 #1047 #1048 #1068 review (non-blocking follow-up)