Fix "ypeError: Cannot call method 'walk' of undefined"

problem with the uglify assetic filter who add option "no-mangle"

bin/uglifyjs

@@ -470,7 +470,7 @@ function normalize(o) {
 
 function getOptions(x, constants) {
     x = ARGS[x];
-    if (x == null) return null;
+    if (!x) return null;
     var ret = {};
     if (x !== "") {
         var ast;