Control plane crashes when backendRef filter is specified in a Route

[sjberman]

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

panic: runtime error: index out of range [0] with length 0

To Reproduce
Steps to reproduce the behavior:

1. Configure a Route with a BackendRef that has a filter.

Expected behavior
No crashes

Your environment

• Version of the NGINX Gateway Fabric
  v2.0.1