infix fun <T> AssertionPlantNullable<T>.toBe(onlyNullAllowed: Nothing?): Unit
Makes the assertion that AssertionPlant.subject is null.
onlyNullAllowed - Has to be null.
AssertionError - Might throw an AssertionError if the assertion made is not correct.
Return
Does not support a fluent API because: what else would you want to assert about null anyway?