Searching by empty string throws ArgumentNullException

[Alexis Sideris]

Searching by empty string :

        var resultsInTitles = this.repo
                                  .All()
                                  .Search(x => x.Title.ToLower())
                                  .Containing(query.ToLower())
                                  .ToRanked()
                                  .OrderByDescending(r => r.Hits);

If query is "" :
Actual -> Throws

Expected -> Return all results

[John Nye]

Thanks for reading an issue @siderisltd. I'll fix this up in the next day or so