Fixed: Preferred word can't have a term that is empty or only spaces

Fixes #218

Co-Authored-By: Mark McDowall <markus101@users.noreply.github.com>

src/Readarr.Api.V1/Profiles/Release/ReleaseProfileController.cs

@@ -1,4 +1,5 @@
 using System.Collections.Generic;
+using System.Linq;
 using FluentValidation;
 using Microsoft.AspNetCore.Mvc;
 using NzbDrone.Common.Extensions;
@@ -32,6 +33,11 @@ namespace Readarr.Api.V1.Profiles.Release
                 {
                     context.AddFailure(nameof(ReleaseProfile.IndexerId), "Indexer does not exist");
                 }
+
+                if (restriction.Preferred.Any(p => p.Key.IsNullOrWhiteSpace()))
+                {
+                    context.AddFailure("Preferred", "Term cannot be empty or consist of only spaces");
+                }
             });
         }