Thanks, I guess my main question is how do I know which are the attributes of a node that lack default values, but maybe the empty text node is a special case that is not allowed at all.
Edit: Ok, empty text nodes are mentioned in the docs.
https://prosemirror.net/docs/guide/#doc.structure
This also means each document has one valid representation. Adjacent text nodes with the same set of marks are always combined together, and empty text nodes are not allowed. The order in which marks appear is specified by the schema.