Prevent NullRef in naming when truncating a null Release Group

(cherry picked from commit 13e29bd257ccfccb09e66c940ffabeb6503c05b5)

src/NzbDrone.Core/Organizer/FileNameBuilder.cs

@@ -327,7 +327,7 @@ namespace NzbDrone.Core.Organizer
         {
             tokenHandlers["{Original Title}"] = m => GetOriginalTitle(movieFile, multipleTokens);
             tokenHandlers["{Original Filename}"] = m => GetOriginalFileName(movieFile, multipleTokens);
-            tokenHandlers["{Release Group}"] = m => Truncate(movieFile.ReleaseGroup, m.CustomFormat) ?? m.DefaultValue("Radarr");
+            tokenHandlers["{Release Group}"] = m => movieFile.ReleaseGroup.IsNullOrWhiteSpace() ? m.DefaultValue("Radarr") : Truncate(movieFile.ReleaseGroup, m.CustomFormat);
         }
 
         private void AddQualityTokens(Dictionary<string, Func<TokenMatch, string>> tokenHandlers, Movie movie, MovieFile movieFile)
@@ -621,6 +621,11 @@ namespace NzbDrone.Core.Organizer
 
         private string Truncate(string input, string formatter)
         {
+            if (input.IsNullOrWhiteSpace())
+            {
+                return string.Empty;
+            }
+
             var maxLength = GetMaxLengthFromFormatter(formatter);
 
             if (maxLength == 0 || input.Length <= Math.Abs(maxLength))