Introduce common support for is-null-safe checks in SpEL nodes

[sbrannen]

Overview

MethodReference and PropertyOrFieldReference already define local isNullSafe() methods, but we now need identical methods in Selection, Projection, and Indexer, and we may potentially need null-safe support for additional operators in the future.

However, doing so without a common mechanism will result in an if-block that's required to check each concrete node type separately.

In other words, the current if-block:

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

Lines 127 to 128 in b1b9ee0

	if ((nextChild instanceof MethodReference methodRef && methodRef.isNullSafe()) ||
	(nextChild instanceof PropertyOrFieldReference pofRef && pofRef.isNullSafe())) {

... would grow to something similar to the following:

if ((nextChild instanceof MethodReference methodRef && methodRef.isNullSafe()) ||
		(nextChild instanceof PropertyOrFieldReference pofRef && pofRef.isNullSafe()) ||
		(nextChild instanceof Selection selection && selection.isNullSafe()) ||
		(nextChild instanceof Projection projection && projection.isNullSafe()) ||
		(nextChild instanceof Indexer indexer && indexer.isNullSafe())) {

Whereas, something unified like the following would be preferred.

if (nextChild.isNullSafe()) {

Prerequisite For

• Introduce null-safe index operator in SpEL #29847
• CompoundExpression omits null-safe syntax in AST string representation of null-safe selection/projection in SpEL #32515