Code Clean up: Convert to null-coalescing operator ?? (#5845)

Co-authored-by: Cody Robibero <cody@robibe.ro>
Co-authored-by: Patrick Barron <18354464+barronpm@users.noreply.github.com>

MediaBrowser.Controller/Library/ItemResolveArgs.cs

@@ -147,10 +147,7 @@ namespace MediaBrowser.Controller.Library
                 throw new ArgumentException("The path was empty or null.", nameof(path));
             }
 
-            if (AdditionalLocations == null)
-            {
-                AdditionalLocations = new List<string>();
-            }
+            AdditionalLocations ??= new List<string>();
 
             AdditionalLocations.Add(path);
         }