fix(frontend): avoid panic when no worker

[yezizp2012]

What's changed and what's your intention?

PLEASE DO NOT LEAVE THIS EMPTY !!!

Please explain IN DETAIL what the changes are in this PR and why they are needed:

As title.

Checklist

• I have written necessary docs and comments
• I have added necessary unit tests and integration tests

Refer to a related PR or issue link (optional)

[codecov]

Codecov Report

Merging #1834 (621b0ea) into main (4f31c18) will decrease coverage by 0.00%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main    #1834      +/-   ##
==========================================
- Coverage   70.76%   70.76%   -0.01%     
==========================================
  Files         607      607              
  Lines       79619    79623       +4     
==========================================
  Hits        56344    56344              
- Misses      23275    23279       +4     

Flag	Coverage Δ
rust	70.76% <0.00%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/frontend/src/scheduler/execution/stage.rs	0.00% <0.00%> (ø)
src/frontend/src/scheduler/query_manager.rs	7.44% <0.00%> (ø)
src/frontend/src/scheduler/worker_node_manager.rs	18.42% <0.00%> (-2.17%)	⬇️

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