Clarify image display when src is an empty string

When src is an empty string (<img src='') and the alt attribute
is not set UAs currently agree that nothing should be displayed.
Update the rules to specify this behaviour.

Closes #3280

source

@@ -25704,6 +25704,9 @@ interface <dfn>HTMLImageElement</dfn> : <span>HTMLElement</span> {
     <p>If the image is <span data-x="img-available">available</span> and the user agent is configured
     to display that image, then the element <span>represents</span> the element's image data.</p>
 
+    <p>If the image has a <code data-x="attr-img-src">src</code> attribute whose value is
+    the empty string, then the element <span>represents</span> nothing.</p>
+
     <p>Otherwise, the element <span>represents</span> the text given by the <code
     data-x="attr-img-alt">alt</code> attribute. User agents may provide the user with a notification
     that an image is present but has been omitted from the rendering.</p>
@@ -121381,6 +121384,7 @@ INSERT INTERFACES HERE
   Rob S,
   Robert Blaut,
   Robert Collins,
+  Robert Hogan,
   Robert Kieffer,
   Robert Millan,
   Robert O'Callahan,