FunctionState should omit empty fields to match the Java nullable fields

[freeznet]

Expected behavior

var state FunctionState = FunctionState{
	ByteValue: []byte("hellopulsar\n"),
	Key:       "test-bytes",
}

The above FunctonState should be convert to JSON {"key":"test-bytes","byteValue":"aGVsbG9wdWxzYXIK"} and submit to pulsar functions worker API.

Actual behavior

The above FunctionState is been sent as JSON {"key":"test-bytes","stringValue":"","byteValue":"aGVsbG9wdWxzYXIK","numberValue":0,"version":0}

Steps to reproduce

When using PutFunctionState with pulsar-admin-go with pulsar 3.2.0, the submitted state will be overwritten by the empty stringValue and lose the byteValue.

System configuration

Pulsar version: 3.2.0