Fix transformed annotation user

[mvasilak]

Fixes issue reported in forum post

When an annotation is transformed, e.g. due to overlapping rects or rects that need to be split, the new annotations are added using document.add. If these annotations have an empty string for the user field, the field is converted to nil. This will cause the PSPDFKIT annotation to be added in the database with createdBy = nil, until the next sync happens, that will assign the proper user.

Guarantying a non-empty user field, alleviates the issue. Alternatively, we can consider adding the username, or even better the userId, in the annotation customData, and not rely at all on user.