Fix to accept empty arrays on Highlight queries

Here is a fix for #2974 specific case.

Tested it with this REST command

<code>
{
   "highlight": {
      "fields": {
         "about": []
      }
   },
   "query": {
      "match": {
        "about": "rock"
      }
   }
}
<code>

and now it's returning the highlighted section as it does with <code>about": {}</code>. In case this could break some other use case please advise.

All tests passed (mvn test -Dtests.filter="@nightly and not(@slow or @backwards)")

Thanks