How should CDI.current() and CDI.getBeanManager() behave for non-CDI apps?

We did hit the following situation:
A user installs a Spring application WAR file in TomEE. In that case we don't boot the CDI container. But the JSF Container calls CDI.current().

How should CDI.current() behave in that case? Throwing an IllegalStateException, returning null or return a non-functional BeanManager?

We should also define the behaviour of CDI.getBeanManager while we are at it.