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
d3c3088c6b