fix($compile): empty normalized href should pass sanitation check

Sometimes IE returns an empty string for its normalized href on a tags.
This should pass the sanitation check in $compile.

Closes angular#2219, angular#2593

src/ng/compile.js

@@ -298,7 +298,7 @@ function $CompileProvider($provide) {
 
           // href property always returns normalized absolute url, so we can match against that
           normalizedVal = urlSanitizationNode.href;
-          if (!normalizedVal.match(urlSanitizationWhitelist)) {
+          if (normalizedVal !== '' && !normalizedVal.match(urlSanitizationWhitelist)) {
             this[key] = value = 'unsafe:' + normalizedVal;
           }
         }