Disallow empty reference labels

lib/rules_block/reference.js

@@ -149,11 +149,16 @@ module.exports = function reference(state, startLine, _endLine, silent) {
     return false;
   }
 
+  label = normalizeReference(str.slice(1, labelEnd));
+  if (!label) {
+    // CommonMark 0.20 disallows empty labels
+    return false;
+  }
+
   // Reference can not terminate anything. This check is for safety only.
   /*istanbul ignore if*/
   if (silent) { return true; }
 
-  label = normalizeReference(str.slice(1, labelEnd));
   if (typeof state.env.references === 'undefined') {
     state.env.references = {};
   }