Cherry-pick #8333 to v1.72.x

[arjan-bal]

Original PR: #8333
Related Issue: #8332

RELEASE NOTES:

• balancer/least_request: fix panic on resolver errors.

[codecov]

Codecov Report

Attention: Patch coverage is 44.44444% with 5 lines in your changes missing coverage. Please review.

Project coverage is 82.22%. Comparing base (edf643f) to head (dcef656).
Report is 1 commits behind head on v1.72.x.

Files with missing lines	Patch %	Lines
balancer/leastrequest/leastrequest.go	44.44%	4 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           v1.72.x    #8339      +/-   ##
===========================================
+ Coverage    82.04%   82.22%   +0.18%     
===========================================
  Files          412      412              
  Lines        40373    40382       +9     
===========================================
+ Hits         33123    33204      +81     
+ Misses        5881     5825      -56     
+ Partials      1369     1353      -16     

Files with missing lines	Coverage Δ
balancer/leastrequest/leastrequest.go	85.24% <44.44%> (-3.25%)	⬇️

... and 27 files with indirect coverage changes

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