[BUG] Deserialization bug in weighted round robin metadata

[anshu1106]

Describe the bug

When weighted round robin weights is deleted for a domain, the weighted routing metadata is set to empty in the cluster state.

Weighted Routing Metadata with weights

"weighted_shard_routing": {
            "awareness": {
                "zone": {
                    "ap-southeast-2b": 1,
                    "ap-southeast-2a": 0,
                    "ap-southeast-2c": 1
                }
            },
            "_version": 2
        }

Weighted Routing Metadata after weights deletion

 "weighted_shard_routing": { "awareness": {}, "_version": 3 }

Due to a bug in deserialization where attributeName is set to null if not present, a NPE is seen.

Caused by: java.lang.NullPointerException: Cannot invoke "String.length()" because "str" is null
        at org.opensearch.core.common.io.stream.StreamOutput.writeString(StreamOutput.java:443)
        at org.opensearch.cluster.routing.WeightedRouting.writeTo(WeightedRouting.java:55)
        at org.opensearch.cluster.metadata.WeightedRoutingMetadata.writeTo(WeightedRoutingMetadata.java:91)
        at org.opensearch.core.common.io.stream.StreamOutput.writeNamedWriteable(StreamOutput.java:1126)
        at org.opensearch.cluster.metadata.Metadata.writeTo(Metadata.java:1145)
        at org.opensearch.cluster.ClusterState.writeTo(ClusterState.java:783)
        at org.opensearch.cluster.coordination.PublicationTransportHandler.lambda$serializeFullClusterState$2(PublicationTransportHandler.java:240)
        at org.opensearch.cluster.coordination.CompressedStreamUtils.createCompressedStream(CompressedStreamUtils.java:42)
        at org.opensearch.cluster.coordination.PublicationTransportHandler.serializeFullClusterState(PublicationTransportHandler.java:238)
        at org.opensearch.cluster.coordination.PublicationTransportHandler$PublicationContext.buildDiffAndSerializeStates(PublicationTransportHandler.java:287)
        at org.opensearch.cluster.coordination.PublicationTransportHandler.newPublicationContext(PublicationTransportHandler.java:233)
        at org.opensearch.cluster.coordination.Coordinator.publish(Coordinator.java:1312)

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To Reproduce

1. Create a 1 or 2 node domain and add weighted round robin weights.
2. Delete weights
3. kill the active master OS process
4. 'ClusterManagerNotDiscovered` exception is seen.
5. Restart OS process
6. NPE is seen in OpenSearch logs

Expected behavior

After removal of weights there should not be any failure in the code

Additional Details

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• OS: [e.g. iOS]
• Version [e.g. 22]

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