`StringUtils.collectionToDelimitedString(…)` fails with `NullPointerException` when the collection contains `null`

[mp911de]

Affects: 5.3.10

After #26316, StringUtils.collectionToDelimitedString(…) behaves slightly different than the previous implementation in 5.3.9.

Calling StringUtils.collectionToDelimitedString(Collections.singleton(null), ",") in version 5.3.10 fails with:

java.lang.NullPointerException
	at org.springframework.util.StringUtils.collectionToDelimitedString(StringUtils.java:1304)
	at org.springframework.util.StringUtils.collectionToDelimitedString(StringUtils.java:1326)

whereas version 5.3.9 returned the string "null".

This ticket is to clarify which behavior is the correct one.

See also spring-projects/spring-data-redis#2167

[bclozel]

Closing in favour of #27419