Existing Wording
If neither data nor a URL is provided, the value should be understood as an assertion that no content for the specified mimeType and/or language is available for the reason stated

Comments
Really? I'd send an Attachment with nothing in it to tell you I can't send one? This doesn't seem terribly useful, and it strikes me that it requires an unnatural behaviour (and extra work) on the part of the receiver.

Grahame's Comments
I don't like it much either. But how else to do it? You could a special flag to say that in addition to not providing url or data, you are not providing them. And that accomplishes what? Else you could impose the requirement that where attachment is used, you also have to have an attribute that says "not available for this language" - it's moving deck chairs around the titanic for what is an edge case. And the current way follows naturally - I have a mime type, and a language, and no content - I guess there isn't any content then