fix(bug): return nil if there are no workers in the requested plugin

[rustatian]

Reason for This PR

closes: #686

Description of Changes

• Return nil instead of error if there are no workers in the requested plugin.

License Acceptance

By submitting this pull request, I confirm that my contribution is made under
the terms of the MIT license.

PR Checklist

[Author TODO: Meet these criteria.]
[Reviewer TODO: Verify that these criteria are met. Request changes if not]

• All commits in this PR are signed (git commit -s).
• The reason for this PR is clearly provided (issue no. or explanation).
• The description of changes is clear and encompassing.
• Any required documentation changes (code and docs) are included in this PR.
• Any user-facing changes are mentioned in CHANGELOG.md.
• All added/changed functionality is tested.

[codecov]

Codecov Report

Merging #699 (42c0d32) into master (a99c14a) will increase coverage by 0.24%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #699      +/-   ##
==========================================
+ Coverage   68.95%   69.19%   +0.24%     
==========================================
  Files          91       91              
  Lines        4422     4422              
==========================================
+ Hits         3049     3060      +11     
+ Misses        974      967       -7     
+ Partials      399      395       -4     

Impacted Files	Coverage Δ
plugins/informer/plugin.go	100.00% <100.00%> (+13.33%)	⬆️
plugins/informer/rpc.go	100.00% <100.00%> (+25.00%)	⬆️
pkg/bst/bst.go	82.85% <0.00%> (+1.42%)	⬆️
plugins/redis/fanin.go	86.66% <0.00%> (+6.66%)	⬆️
plugins/websockets/pool/workers_pool.go	77.08% <0.00%> (+8.33%)	⬆️

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