You can return null from a @Bean method and it does some clever stuff internally using a final package private class (which is therefore inaccessible to users) called NullBean. The result is just a bean that cannot satisfy any dependencies. It would make sense if the functional form of the same bean could do the same trick, either returning null from the Supplier or some other contract that allows the BeanFactory to behave the same way.
#21572 Consistent non-exposure of null beans in the BeanFactory API
#21601 Use available type information in functional bean definitions for resolving FactoryBean type