Show empty state message for unassigned devices snapshots page

[cstns]

Description

This scenario was missed when I added the empty state message for instance assigned devices

Related Issue(s)

part of #5111

Checklist

• I have read the contribution guidelines
• Suitable unit/system level tests have been added and they pass
• Documentation has been updated
  • Upgrade instructions
  • Configuration details
  • Concepts
• Changes flowforge.yml?
  • Issue/PR raised on FlowFuse/helm to update ConfigMap Template
  • Issue/PR raised on FlowFuse/CloudProject to update values for Staging/Production

Labels

• Includes a DB migration? -> add the area:migration label

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.27%. Comparing base (ee71af9) to head (ae69ae4).
Report is 1 commits behind head on add-device-pipeline-audit-log-event.

Additional details and impacted files

@@                         Coverage Diff                          @@
##           add-device-pipeline-audit-log-event    #5157   +/-   ##
====================================================================
  Coverage                                78.27%   78.27%           
====================================================================
  Files                                      339      339           
  Lines                                    15989    15989           
  Branches                                  3698     3698           
====================================================================
  Hits                                     12515    12515           
  Misses                                    3474     3474           

Flag	Coverage Δ
backend	78.27% <ø> (ø)

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