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[bot]]

Review Checklist

Hello reviewers! :wave: Please follow this checklist when reviewing this Pull Request.

General

• [ ] Ensure that the Pull Request has a descriptive title.
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• [ ] vtctl command output order should be stable and awk-able.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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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