Spring Resolves gh-25140

Simple fix for GH-25140

Just to be sure, the call

multiValueMap.addAll("some-key", null);

will fail with NullPointer.

multiValueMap.addAll("some-other-key", Collections.emptyList());

produces

multiValueMap.getFirst("some-other-key") == null

Comment From: midumitrescu

already resolved by @jhoeller with another commit