Clarify behavior of the Elvis SpEL operator in documentation (#30352)

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

spring-expression/src/main/java/org/springframework/expression/spel/ast/Elvis.java

@@ -26,9 +26,9 @@ import org.springframework.util.Assert;
 import org.springframework.util.ObjectUtils;
 
 /**
- * Represents the elvis operator <code>?:</code>. For an expression <code>a?:b</code> if <code>a</code> is not null,
- * the value of the expression is <code>a</code>, if <code>a</code> is null then the value of the expression is
- * <code>b</code>.
+ * Represents the Elvis operator <code>?:</code>. For an expression <code>a?:b</code> if <code>a</code> is neither null
+ * nor an empty String, the value of the expression is <code>a</code>.
+ * If <code>a</code> is null or the empty String, then the value of the expression is <code>b</code>.
  *
  * @author Andy Clement
  * @author Juergen Hoeller
@@ -43,8 +43,8 @@ public class Elvis extends SpelNodeImpl {
 
 
 	/**
-	 * Evaluate the condition and if not null, return it.
-	 * If it is null, return the other value.
+	 * Evaluate the condition and if neither null nor an empty String, return it.
+	 * If it is null or an empty String, return the other value.
 	 * @param state the expression state
 	 * @throws EvaluationException if the condition does not evaluate correctly
 	 * to a boolean or there is a problem executing the chosen alternative