From what I can tell, the language for link labels specifies a maximum number of characters (999), but not a minimum. Thus, it seems an empty link label should technically be allowed. The reference implementation does not interpret
 as a link label. In particular, this is rendered as plain text:
This seems reasonable behavior to me. Otherwise, at least one of the problems would be that
[link] would be ambiguous. But I think this could perhaps be spelled out in the spec.