I don't think there's a clearcut answer to this question, and the value a cataloger chooses will depend somewhat on the artist's intent and user expectations. In the case of artists' books, I would lean toward "three-dimensional form" as the content type for a book that is entirely blank and coding the type of record as "a" for books. If there were the tiniest bit of text, for example, an autograph or numbering indicating edition size by the book artist, I might consider "text" as the content type. If the book artist highlighted the artifactual nature or "object-ness" of the book in an artist's statement, that would be sound evidence for going with "three-dimensional form."
I've been following this discussion on the PCC list, and I know that one person suggested that the RDA Examples Working Group add an example of this case on the RDA content type element page. I chair this group, and it's something we can discuss. However, the challenge of adding edge cases as examples is that they may not apply in every context, it's hard to convey concisely the nuance of the particular situation, and it risks being overly prescriptive. One of the best things about our discussion lists is that we can talk through the pros and cons of nuanced cases together and work our way toward best practices without necessarily landing on one solution for all.
Re: your question about adding a "no content" content type, I think this is probably unnecessary. Content type in RDA is defined as "A categorization that reflects the fundamental form of communication in which the content is expressed and the human sense through which it is intended to be perceived." I have a hard time imagining a resource in our collections that couldn't be perceived in some way through at least one of our senses.
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Jessica Grzegorski
Rare Materials Metadata Librarian
Northwestern University Libraries
She/Her/Hers
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