Fix crash when BackendRef filter is specified

[sjberman]

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.

Closes #3507

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

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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 86.82%. Comparing base (6752eaa) to head (0e7bb61).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3508      +/-   ##
==========================================
+ Coverage   86.79%   86.82%   +0.02%     
==========================================
  Files         127      127              
  Lines       15079    15079              
  Branches       62       62              
==========================================
+ Hits        13088    13092       +4     
+ Misses       1840     1836       -4     
  Partials      151      151              

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