Should empty property data type references be valid? #2417

The question of whether a prefix-only property data type value is valid came up earlier, and we clarified that it is allowed (in keeping with CURIEs).

But how do we square this with requiring a meta element's property attribute to reference a property? By our current definition, the following is valid: <meta property="foo:">bar</meta>

The only place a "URL-only" prefix makes sense in a scheme attribute. Everything else requires a reference value (i.e., the property).

Do we have to go back through each attribute and clarify the ones that have to have a reference at least one character in length, or should we go back to the way things were when we expected all attribute values to be an optional prefix followed by a reference? (i.e., say that although an empty path-relative-scheme-less-URL string is valid, it is not a valid reference)