[release-1.20] Fix eds cluster loadassignment with invalid endpoint

[istio-testing]

Manual cherrypick required.

#50690 failed to apply on top of branch "release-1.20":

Applying: fix eds cluster has invalid endpoints
Using index info to reconstruct a base tree...
M	pilot/pkg/xds/endpoints/endpoint_builder.go
Falling back to patching base and 3-way merge...
Auto-merging pilot/pkg/xds/endpoints/endpoint_builder.go
CONFLICT (content): Merge conflict in pilot/pkg/xds/endpoints/endpoint_builder.go
error: Failed to merge in the changes.
hint: Use 'git am --show-current-patch=diff' to see the failed patch
Patch failed at 0001 fix eds cluster has invalid endpoints
When you have resolved this problem, run "git am --continue".
If you prefer to skip this patch, run "git am --skip" instead.
To restore the original branch and stop patching, run "git am --abort".

[zirain]

fixed