Null paths throw exceptions with "as" syntax

[dfreedm]

If a template is bound to a path that would evaluate an undefined model member, then an exception is thrown at https://github.com/Polymer/polymer-expressions/blob/master/src/polymer-expressions.js#L149

Example:

  <template id="t" bind="{{ foo.bar as b }}">
    <span>{{ b }}</span>
  </template>
  <script>
    t.bindingDelegate = new PolymerExpressions();
    t.model = {};
  </script>

[rafaelw]

Fixed: db4a055