Handle null for parent name with clean path is empty

(cherry picked from commit bd3816dac9b540d2f95960a254bb21ab3c91421e)

src/NzbDrone.Common/Extensions/PathExtensions.cs

@@ -101,6 +101,11 @@ namespace NzbDrone.Common.Extensions
         {
             var cleanPath = childPath.GetCleanPath();
 
+            if (cleanPath.IsNullOrWhiteSpace())
+            {
+                return null;
+            }
+
             return Directory.GetParent(cleanPath)?.Name;
         }