Fix warnings, improve performance (#1665)

* Fix warnings, improve performance

`QueryResult.Items` is now a `IReadOnlyList` so we don't need to
allocate a new `Array` when we have a `List` (and `Items` shouldn't need to
be mutable anyway)

* Update Providers .csproj to latest C#

* Remove extra newline from DtoService.cs

* Remove extra newline from UserLibraryService.cs

MediaBrowser.Model/Querying/QueryResult.cs

@@ -1,3 +1,6 @@
+using System;
+using System.Collections.Generic;
+
 namespace MediaBrowser.Model.Querying
 {
     public class QueryResult<T>
@@ -6,7 +9,7 @@ namespace MediaBrowser.Model.Querying
         /// Gets or sets the items.
         /// </summary>
         /// <value>The items.</value>
-        public T[] Items { get; set; }
+        public IReadOnlyList<T> Items { get; set; }
 
         /// <summary>
         /// The total number of records available
@@ -16,7 +19,7 @@ namespace MediaBrowser.Model.Querying
 
         public QueryResult()
         {
-            Items = new T[] { };
+            Items = Array.Empty<T>();
         }
     }
 }