Update the definition of the content of the @longdesc attribute to match the requirements for the @src attribute.
Referring to its definition in HTML5, the HTML Image Description Extension spec currently says that the content of the @longdesc attribute MUST be:
]] a valid URL potentially surrounded by spaces [[
This definition, however, is too loose. For contrast, the definition of the img element’s @src attribute pointes to HTML5’s definition of 'a validt non-empty URL'. Thus, @src is required to be: 
]] a valid non-empty URL potentially surrounded by spaces [[
WHY CURRENT SITUATION IS DANGEROUS:
The consequence of the above is that the HTML5 validator whines if the @src attribute only contains the empty string (including the empty string surrounded by spaces) - which is nice. But, conversely, if the @longdesc attribute contains the empty string (potentially surrounded by spaces), the validator does (currently) not whine.
It is also worth mentioning that in the infamous Longdesc Lottery, then "is blank" (followed by "is not a valid URL"), is the first mentioned example of longdesc misuse. When possible, the HTML Image Description Extension spec should demonstrate that it tries to avoid the issues that the Longdesc Lottery article takes up.
As part of this bug fix, please do as well replace the following text - this:
]] If a longdesc attribute has invalid content, [[
With the following text:
]] If the content of a longdesc attribute is not a
" >valid non-empty-url</a> [[
I agree. I'll update this in the next draft. (I thought I had this already, actually)
Fixed as of https://dvcs.w3.org/hg/html-proposals/raw-file/0dd2e510d4e1/longdesc1/longdesc.html