fix($compile): don't look for class directives in empty string

if className is undefined or empty string, don't bother looking for directives in there

src/ng/compile.js

@@ -474,7 +474,7 @@ function $CompileProvider($provide) {
 
           // use class as directive
           className = node.className;
-          if (isString(className)) {
+          if (isString(className) && className !== '') {
             while (match = CLASS_DIRECTIVE_REGEXP.exec(className)) {
               nName = directiveNormalize(match[2]);
               if (addDirective(directives, nName, 'C', maxPriority)) {