title: Query if field equals value or is null
body: <p>I am trying to filter a collection by a field value and also get all entries where the field is equal to null. Since the in operator does not support null values, I can not take the simple route and query for:</p>
  <pre><code>where('field', 'in', ['value', null])
  </code></pre>
  <p>I am currently solving this issue by having two queries. One getting the entries where the field is null and one getting the entries where the field equals the value:</p>
  <pre><code>where('field', '==', 'value')
  where('field', '==', null)
  </code></pre>
  <p>But this returns me more entries than necessary, and I have to sort and merge the entries client side. I would rather have a single query return me all entries, so I can do things like pagination across the combined list, rather than paginating over both list separately.</p>
  <p>Is there any method to accomplish this?</p>