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

src/Sonarr.Api.V3/Profiles/Release/ReleaseProfileModule.cs

@@ -1,4 +1,5 @@
 using System.Collections.Generic;
+using System.Linq;
 using FluentValidation;
 using FluentValidation.Results;
 using NzbDrone.Common.Extensions;
@@ -36,6 +37,11 @@ namespace Sonarr.Api.V3.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");
+                }
             });
         }