`plone.api.relation.create` error on not required RelationList field

[szakitibi]

Steps to reproduce

• Provide a behavior or extend a schema with a not required RelationList field:

    some_related_stuff = RelationList(
        title="Some not required related stuff",
        value_type=RelationChoice(vocabulary='plone.app.vocabularies.Catalog'),
        required=False,
    )

• It is important not to set default=[], which according to the documentation is a dangerous default. (IRelatedItems has it 10 years now though?!?)
• Create an object obj1 with having no some_related_stuff relations, aka None value.
• Try to create a new relation using plone.api.relation.create.

api.relation.create(source=obj1, target=obj2, relationship='some_related_stuff')

What happens

It goes into an error, since it gets None here:

https://github.com/plone/plone.api/blob/08884b755450cdc5290d640b826d3f659025179f/src/plone/api/relation.py#L220

What I expect

A relation created.