Fix crash when BackendRef filter is specified (#3508)

[sjberman]

Cherrypick of #3508

Problem: When a BackendRef filter is specified in a Route, the control plane crashes due to an index out of range error.

Solution: Fix the way we build the filter list so we don't access an undefined index.

Testing: Verified that the crash no longer occurs.

Checklist

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• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto main
• I will ensure my PR is targeting the main branch and pulling from my branch from my own fork

Release notes

If this PR introduces a change that affects users and needs to be mentioned in the release notes,
please add a brief note that summarizes the change.

Fix crash when BackendRef filter is specified.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.79%. Comparing base (dd96a01) to head (d6a3565).
Report is 1 commits behind head on release-2.0.

Additional details and impacted files

@@               Coverage Diff               @@
##           release-2.0    #3575      +/-   ##
===============================================
+ Coverage        86.74%   86.79%   +0.04%     
===============================================
  Files              127      127              
  Lines            15079    15079              
  Branches            62       62              
===============================================
+ Hits             13081    13088       +7     
+ Misses            1845     1840       -5     
+ Partials           153      151       -2     

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