scheduler: optimize when only 1 reservation/node

[ZiMengSheng]

Ⅰ. Describe what this PR does

Ⅱ. Does this pull request fix one issue?

Ⅲ. Describe how to verify it

Ⅳ. Special notes for reviews

V. Checklist

• I have written necessary docs and comments
• I have added necessary unit tests and integration tests
• All checks passed in make test

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 67.92%. Comparing base (290ae09) to head (d639228).

Files	Patch %	Lines
pkg/scheduler/plugins/reservation/nominator.go	50.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2136      +/-   ##
==========================================
+ Coverage   67.91%   67.92%   +0.01%     
==========================================
  Files         441      441              
  Lines       41547    41551       +4     
==========================================
+ Hits        28215    28223       +8     
+ Misses      10902    10896       -6     
- Partials     2430     2432       +2     

Flag	Coverage Δ
unittests	67.92% <50.00%> (+0.01%)	⬆️

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[saintube]

/lgtm

[ZiMengSheng]

/approve

[koordinator-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ZiMengSheng

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