Support ports with default values

[adivardi]

Is your feature request related to a problem? Please describe.
Currently, Groot cannot be used for nodes with default port values.
For such nodes, Groot will either:

• Explicitly set the default value, if available in the models xml - OK, but requires propel models xml, and is tedious for repeated or complex types
• Set the port value to "" - crashes for any non-string port

So after every save in Groot, we have to manually remove these ports.

Describe the solution you'd like
According to the tutorials, ports with default values should be omitted from the XML. I think either a solution can be either:

• Groot can properly detects a port with default value and excludes it from the xml unless a value is given
• A special string value indicating this port should be omitted (e.g. N/A)

Obviously, I think the first solution is nicer.

Describe alternatives you've considered
See above

Additional context