This commit improves both the javadoc and the reference guide section on
the Elvis SpEL operator to clarify that in addition to null objects,
empty Strings also lead the operator to evaluate to its second operand.
The reference guide's advanced snippet is modified to use such an empty
String instead of null to make that behavior prominent with some code.
See gh-30318 Closes gh-30352
Comment From: simonbasle
draft while waiting for reference guide changes that we might need to adapt to / rebase on.
Comment From: simonbasle
Rebased on top of Antora changes, ready for merge.