fixup! Check for nullity using "is null" or "is object"

src/FakeItEasy/ServiceLocator.cs

@@ -16,7 +16,7 @@ static ServiceLocator()
         /// <typeparam name="T">Type of the service.</typeparam>
         /// <returns>An instance of the service type.</returns>
         [DebuggerStepThrough]
-        internal static T Resolve<T>()
+        internal static T Resolve<T>() where T : class
         {
             var service = Service<T>.Instance;
             return service is null