Remove placeholder in non-empty Chips component

[KeithGillette]

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Current behavior
<p-chips placeholder="some text"> displays some text even when chips have been added.

Expected behavior
Placeholder text should only appear in an empty Chips component.

Minimal reproduction of the problem with instructions
http://plnkr.co/edit/Yebwt4Jo8JFQhRRmBYWh?p=preview

What is the motivation / use case for changing the behavior?
Inconsistent with other components that support placeholder text. See similar case made & implemented for <p-autocomplete [multiple]="true">