restore input to default if no element is selected

[davidcv5]

The paper-dropdown-menu-light shows label out of place when an item is selected and then all items are removed. i.e. Binding items to a firebase-query or any other dynamic source.

Expected outcome

paper-dropdown-menu-light should return to its default state if all content is removed.

Actual outcome

After items get deleted, the innerHTML of <div id="input"... is empty, instead of  , which makes the div smaller and causes the label to show out of place.

Steps to reproduce

1. Put a paper-dropdown-menu-light on the page.
2. Put a paper-menu inside with class="dropdown-content".
3. Put a <template is="dom-repeat" .. inside with items pointing to an array
4. Select an element from the dropdown.
5. Remove all elements from the array using the corresponding array mutation methods.

Test case: https://jsbin.com/qufitil/edit?html,output