Introduce isNullSafe() in SpelNodeImpl

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

To address the common need for an is-null-safe check, this commit
introduces an isNullSafe() method in SpelNodeImpl with a default
implementation that returns false.

Closes gh-32516

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2023 the original author or authors.
+ * Copyright 2002-2024 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -24,7 +24,6 @@
 import org.springframework.expression.spel.CodeFlow;
 import org.springframework.expression.spel.ExpressionState;
 import org.springframework.expression.spel.SpelEvaluationException;
-import org.springframework.expression.spel.SpelNode;
 
 /**
  * Represents a DOT separated expression sequence, such as
@@ -120,14 +119,13 @@ public String toStringAST() {
 		for (int i = 0; i < getChildCount(); i++) {
 			sb.append(getChild(i).toStringAST());
 			if (i < getChildCount() - 1) {
-				SpelNode nextChild = getChild(i + 1);
+				SpelNodeImpl nextChild = this.children[i + 1];
+				if (nextChild.isNullSafe()) {
+					sb.append("?.");
+				}
 				// Don't append a '.' if the next child is an Indexer.
 				// For example, we want 'myVar[0]' instead of 'myVar.[0]'.
-				if (!(nextChild instanceof Indexer)) {
-					if ((nextChild instanceof MethodReference methodRef && methodRef.isNullSafe()) ||
-						(nextChild instanceof PropertyOrFieldReference pofRef && pofRef.isNullSafe())) {
-						sb.append('?');
-					}
+				else if (!(nextChild instanceof Indexer)) {
 					sb.append('.');
 				}
 			}

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2023 the original author or authors.
+ * Copyright 2002-2024 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -77,6 +77,7 @@ public MethodReference(boolean nullSafe, String methodName, int startPos, int en
 	 * Does this node represent a null-safe method reference?
 	 * @since 6.0.13
 	 */
+	@Override
 	public final boolean isNullSafe() {
 		return this.nullSafe;
 	}

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

@@ -76,6 +76,7 @@ public PropertyOrFieldReference(boolean nullSafe, String propertyOrFieldName, in
 	/**
 	 * Does this node represent a null-safe property or field reference?
 	 */
+	@Override
 	public boolean isNullSafe() {
 		return this.nullSafe;
 	}

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

@@ -181,6 +181,16 @@ public int getEndPosition() {
 		return this.endPos;
 	}
 
+	/**
+	 * Determine if this node is the target of a null-safe navigation operation.
+	 * <p>The default implementation returns {@code false}.
+	 * @return {@code true} if this node is the target of a null-safe operation
+	 * @since 6.1.6
+	 */
+	public boolean isNullSafe() {
+		return false;
+	}
+
 	/**
 	 * Check whether a node can be compiled to bytecode. The reasoning in each node may
 	 * be different but will typically involve checking whether the exit type descriptor