Empty Context Keys in Transaction Serialization

The Agent will produce empty context object for transaction context when it's not set. This is due to the pattern of empty objects being set

https://github.com/elastic/apm-agent-nodejs/blob/ba570ba2af5243fa99cc1a36421cc2eb5ea930c3/lib/instrumentation/transaction.js#L154-L155

Would is be safe to change this behavior to _not_ serialize the property if it's unset/null/false instead of setting an empty object? If so, we should change it.