title: Return null for FirstOrDefault() on empty IEnumerable<int>?
body: <p>Say I have the following snippet:</p>
  <pre><code>int? nullableId = GetNonNullableInts().FirstOrDefault();
  </code></pre>
  <p>Because <code>GetNonNullableInts()</code> returns integers, the <code>FirstOrDefault</code> will default to <code>0</code>.<br />
  Is there a way to make the <code>FirstOrDefault</code> on a list of integers return a <code>null</code> value when the list is empty?</p>