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.tests/ValidatorTests.cs

@@ -45,6 +45,17 @@ internal sealed class ValidatorTests
 		}
 	}
 
+	[Test]
+	public async Task I_can_validate_with_a_null_instance_without_throwing()
+	{
+		var validator = new PropertyValidator<Person?, string?>(person
+			=> person!.Name, rule => rule.Must(value => value is not null, "Name is required."));
+
+		var result = validator.Validate((Person?)null);
+
+		await Assert.That(result.IsValid).IsTrue();
+	}
+
 	[Test]
 	public async Task I_can_compose_nested_validators_and_aggregate_property_paths()
 	{

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";