Handle String() with no arguments.

Fix #91

lib/compress.js

@@ -1566,6 +1566,9 @@ merge(Compressor.prototype, {
                     }
                     break;
                   case "String":
+                    if (self.args.length == 0) return make_node(AST_String, self, {
+                        value: ""
+                    });
                     return make_node(AST_Binary, self, {
                         left: self.args[0],
                         operator: "+",