Perl translation

There's a few missing C# test cases

[Test]
public void NullSequence()
{
  Assert.That(Kata.UniqueInOrder(null), Throws.InstanceOf<ArgumentNullException>());
  // OR (undefined requirement)
  Assert.That(Kata.UniqueInOrder(null), Is.Empty)
}

[Test]
public void NullValue()
{
  // Breaks solutions that use default as a placeholder value
  Assert.That(Kata.UniqueInOrder(new string[]{"Hello", null, null, "world"}), Is.EqualTo(new string[]{"Hello", null, "world"}));
}

[Test]
public void LeadingZero()
{
  // Breaks solutions that use default as a placeholder value
  Assert.That(Kata.UniqueInOrder(new int[]{0, 1, 2, 3}), Is.EqualTo(new int[]{0, 1, 2, 3}));
}

[Test]
public void EmptyIntegerArray()
{
  // Breaks solutions that use default as a placeholder value
  Assert.That(Kata.UniqueInOrder(Array.Empty<int>()), Is.EqualTo(Array.Empty<int>()));
}

[Test]
public void MixedTypes()
{
  // Make sure solutions don't use assumptions between types
  Assert.That(Kata.UniqueInOrder(new object[]{1, "2", 2, 2, "5", "5"}), Is.EqualTo(new object[]{1, "2", 2, "5"}));
}

private class Foo
{
  public override bool Equals(object other) => other is Foo || other is Bar;
}

private class Bar
{
  public override bool Equals(object other) => other is Foo || other is Bar;
}

[Test]
public void RemovesDuplicateCustomTypes()
{
  // Make sure solutions don't use assumptions between types
  Assert.That(Kata.UniqueInOrder(new object[]{new Foo(), 1, new Bar(), new Foo(), string.Empty}).Select(res => res.GetType()), Is.EqualTo(new object[]{typeof(Foo), typeof(1), typeof(Bar), typeof(string)}));
}