nunit: A bug when using Assert.That() with Is.Not.Empty

I’ve noticed some unexpected behaviour when I use the following code:

// EXPECTED - throws NUnit.Framework.AssertionException
Assert.That(null, Is.Not.Null);

// UNEXPECTED - throws System.ArgumentException
Assert.That(null, Is.Not.Empty);

// EXPECTED - throws NUnit.Framework.AssertionException
Assert.That("", Is.Not.Empty)

// UNEXPECTED - throws System.ArgumentException
Assert.That(null, Is.Not.Null.Or.Empty)

// UNEXPECTED - doesn't throw exception at all
Assert.That("", Is.Not.Null.Or.Empty)

Note that in all cases I would expect NUnit.Framework.AssertionException to be thrown.

I guess this is a bug and I’m wondering if the code marked with // UNEXPECTED has been unit tested?