Prior to this commit, RestTemplate and HttpMessageConverterExtractor did
not validate that the supplied HttpMessageConverter list contained no
null elements, which can lead to a NullPointerException when the
converters are accessed.
This commit improves the user experience by failing immediately if the
supplied HttpMessageConverter list contains a null element. This applies
to constructors for RestTemplate and HttpMessageConverterExtractor as
well as to RestTemplate#setMessageConverters().
Note, however, that RestTemplate#getMessageConverters() returns a mutable
list. Thus, if a user modifies that list so that it contains null values,
that will still lead to a NullPointerException when the converters are
accessed.
This commit also introduces noNullElements() variants for collections in
org.springframework.util.Assert.
Closes gh-23151
4000b244ff