[release-1.15] fix empty endpoints weight for AUTO_PASSTHROUGH gateway

[bincherry]

Please provide a description of this PR:

Manual cherrypick of #40243

This cannot be done by a bot because there are different struct names on the two branches, according to #40226 , which changes LocLbEndpointsAndOptions to LocalityEndpoints on master branch.

[istio-testing]

Hi @bincherry. Thanks for your PR.

I'm waiting for a istio member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

I understand the commands that are listed here.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[zirain]

/ok-to-test

[bincherry]

It seems that something wrong with the arm64 testing. Need some help please.

[zirain]

cc @howardjohn arm64 is supported on v1.15?

[ericvn]

/retest