Minor code style tweaks

Author: Sergio0694

Microsoft.Toolkit/Collections/ObservableGroupedCollection.cs

@@ -44,7 +44,7 @@ internal bool TryGetList([NotNullWhen(true)] out List<ObservableGroup<TKey, TVal
         {
             list = Items as List<ObservableGroup<TKey, TValue>>;
 
-            return !(list is null);
+            return list is not null;
         }
     }
 }

Microsoft.Toolkit/Diagnostics/Guard.cs

@@ -68,7 +68,7 @@ public static void IsNull<T>(T? value, string name)
         public static void IsNotNull<T>([NotNull] T? value, string name)
             where T : class
         {
-            if (!(value is null))
+            if (value is not null)
             {
                 return;
             }
@@ -88,7 +88,7 @@ public static void IsNotNull<T>([NotNull] T? value, string name)
         public static void IsNotNull<T>([NotNull] T? value, string name)
             where T : struct
         {
-            if (!(value is null))
+            if (value is not null)
             {
                 return;
             }
@@ -196,7 +196,7 @@ public static void IsAssignableToType<T>(object value, string name)
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         public static void IsNotAssignableToType<T>(object value, string name)
         {
-            if (!(value is T))
+            if (value is not T)
             {
                 return;
             }