Used to assert that one value is equal to null.
The value that should equal null.
The exception message.
The file name that the assert failed in. Should be left as default.
The file line that the assert failed in. Should be left as default.
If value is NOT null, will throw an UnitTestException.
// Will throw an exception like "checkit.exception.UnitTestException@example.d(6): Expected <null>, got <blah>." string test = "blah"; test.shouldBeNull();