Check if DatePicker value is null
<p>I would like to check if the value of a <code>DatePicker</code> is null (== no date in the box).</p>

<p>By default the <code>Text</code> of my <code>DatePicker</code> is set to something like <code>Select a date</code>, so I can't use the <code>Text</code> property to check.</p>