e2e framework change to find empty throttler config

[harshit-gangal]

Description

This PR will stop emitted config not found logs from throttler for e2e test in vttablet.

Example:

throttler.go:592] Throttler.retryReadAndApplyThrottlerConfig(): error reading throttler config. Will retry in 10s. Err=node doesn't exist: keyspaces/vttest/SrvKeyspace

Related Issue(s)

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

[vitess-bot]

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[shlomi-noach]

thank you

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.38%. Comparing base (f6067e0) to head (3c55db3).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17228      +/-   ##
==========================================
- Coverage   67.40%   67.38%   -0.02%     
==========================================
  Files        1570     1570              
  Lines      252903   252903              
==========================================
- Hits       170460   170410      -50     
- Misses      82443    82493      +50     

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