SpringBoot ConditionMessage#items throws an NPE with a null list of items although the Javadoc states it is tolerated

Hi,

as noted in #22330 the several items methods in ConditionMessage state that the passed items might be null.

* @param items the source of the items (may be {@code null})

Yet, currently they would thrown a NPE if null would be passed. This PR fixes that.

Cheers, Christoph