Messages: mandatory empty string values should still return length

[JurgenR]

The length should still be serialized if a string is mandatory but None is passed

[JurgenR]

I do not remember in which message I encountered this but looking at the default values it's confusing as to what happens to the value. This should be clarified

[JurgenR]

For serialization there is nothing preventing from passing None to a mandatory field. There's a couple of possible solutions:

• Adapt all string fields with a default empty string. However doing that will cause them to be longer mandatory and causes other fields to have to have defaults as well
• Assign a default to each type and have it returned instead of None
• Make an exception case for string type

The last 2 options could possibly have a performance impact