fix(validation): return no problems for null reference instance in Rule.Validate

Rule<T,TProperty>.Validate cast a null instance to T and validated it, so a
reference T whose accessor dereferences the instance threw
NullReferenceException on a direct validator.Validate(context) with a null
instance. Guard the null-instance path and return no problems instead.

src/request.validation/Rule.cs

@@ -46,7 +46,7 @@ internal abstract record Rule<T, TProperty> : Rule
 
 		if (context.Instance is null && default(T) is null)
 		{
-			return Validate((T)context.Instance!, context);
+			return [];
 		}
 
 		var actualType = context.Instance?.GetType().FullName ?? "null";