[DevKit HOWTO_08] sign request in lib does produce empty ftPyItemId field for the tip pay item entry - should be undefined if empty

[turtletramp]

The ftPayItemId is undefined in the Tip pay item in the response of /pay endpoint.
But when executing HOWTO_08 this field is an empty string which then also results in an error in the /sign request:

TODO:

• analyse why we have an empty string "" in the /sign request --> workaround implemented to be sure it is null instead of ""
• analyse why the /sign response contains the JSON de-serialization error --> after the entries with "" is null it works
• fix issues when root cause(s) is/are clear ✅