ReplacePrefixMatch in the URLRewrite filter does not handle empty match prefix correctly

*Description*:

In its [examples](https://gateway-api.sigs.k8s.io/reference/spec/#gateway.networking.k8s.io%2fv1.HTTPPathModifier), the Gateway API uses "foo" in "Prefix Match". However, we can conclude that the `HTTPPathModifier` should also work on `/` as the path prefix match.

The PR #5254 PR demonstrates that `/` is not handled correctly.

*Repro steps*:

Follow the end-2-end test, which is added in #5254

*Environment*:

In our environment, we use `docker.io/envoyproxy/gateway:v1.3.0`. The bug is reproducible in the `main` branch, and is reproducible in `v1.2.5`