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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