fix($parse): mark empty expressions as constants and literals

In response of [#1405 (comment)](https://github.com/angular/angular.js/issues/1405#issuecomment-45525863)

I decided to mark it as a literal because `$parse('undefined')` is a literal.
I didn't want to add an extra code path in the `Parser`, that will be checked every time an expression is being parsed, so I decided to put it straight in the cache