xdsclient: skip localities with zero weight when parsing the resource

[easwars]

We do skip localities with zero weight here:

grpc-go/xds/internal/balancer/clusterresolver/configbuilder.go

Line 254 in 28de486

if locality.Weight == 0 {

But, we have no tests for this.

[markdroth]

FWIW, in C-core, we skip these localities inside of XdsClient when parsing the EDS resource. They never get returned from the XdsClient, so there's no need to deal with them in any of the LB policies.