A little Monday cleanup (#13823)

* Introduce root key constant

* Do not enforce ProblemDetails on every non-success response from the API + remove invalid NotFoundResult response types from various endpoints

* Update OpenAPI JSON contract to reflect the new NotFound results

src/Umbraco.Core/Constants-System.cs

@@ -18,6 +18,11 @@ public static partial class Constants
         /// <remarks>Use this instead of re-creating the string everywhere.</remarks>
         public const string RootString = "-1";
 
+        /// <summary>
+        ///     The GUID identifier for global system root node.
+        /// </summary>
+        public static readonly Guid? RootKey = null;
+
         /// <summary>
         ///     The integer identifier for content's recycle bin.
         /// </summary>