Spring Replace hardcoded "null" with NULL_STRING constant in ObjectUtils.nullSafeConciseToString()

Description:

This pull request replaces the hardcoded "null" string in the nullSafeConciseToString method with the existing NULL_STRING constant to improve maintainability and readability.

Changes:

Replaced occurrences of the hardcoded "null" string with the already declared NULL_STRING constant.

Comment From: sbrannen

Good catch!

That was an oversight on my part.

This has been merged into main.

Thanks