title: Sorting by multiple fields (collation)
body: <p>In my objectbox model class I have 2 fields:</p>
  <pre><code>@Convert(converter = LocalDateTimeConverter.class, dbType = String.class)
  public LocalDateTime dateModified;
  @Convert(converter = LocalDateTimeConverter.class, dbType = String.class)
  public LocalDateTime dateTaken;
  </code></pre>
  <p>Some elements have both fields filled while other have dateModified not null and dateTaken is null. I'd like to sort query results by dateTaken if it's not null, otherwise by dateModified. Is it possible to do so with Objectbox query (i.e. some sort of collation)?</p>