Internal representation of a null bean instance, e.g. for a
null value
returned from
FactoryBean.getObject() or from a factory method.
Each such null bean is represented by a dedicated NullBean instance
which are not equal to each other, uniquely differentiating each bean as returned
from all variants of BeanFactory.getBean(java.lang.String).
However, each such instance will return true for #equals(null)
and returns "null" from #toString(), which is how they can be tested
externally (since this class itself is not public).