Failure to emit empty NETCONF-acquired data in JSON

When we source data from NETCONF, the inbound codec emits an empty string for each empty element, overriding EmptyCodec's result (which is always null).

This is incorrect behavior, which is manifested on egress in JSON writer, which ends up seeing a String instance where null is expected. Rather than adding a special case to the writer, fix ingress code structure to correctly assign value.