Include '?' for null-safe navigation in SpEL AST representations

Prior to this commit, if a Spring Expression Language (SpEL) expression
contained property, field, or method references using the null-safe
navigation operator (?.), the generated AST String representation
incorrectly omitted the '?' characters.

For example, 'myProperty?.myMethod()' had a generated AST string
representation of 'myProperty.myMethod()'.

This commit addresses this by introducing isNullSafe() in
MethodReference and reworking the logic in
CompoundExpression.toStringAST().

Closes gh-31326

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

@@ -28,7 +28,9 @@
 
 /**
  * Represents a DOT separated expression sequence, such as
- * {@code property1.property2.methodOne()}.
+ * {@code property1.property2.methodOne()} or
+ * {@code property1?.property2?.methodOne()} when the null-safe navigation
+ * operator is used.
  *
  * <p>May also contain array/collection/map indexers, such as
  * {@code property1[0].property2['key']}.
@@ -122,6 +124,10 @@ public String toStringAST() {
 				// 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('?');
+					}
 					sb.append('.');
 				}
 			}

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

@@ -50,6 +50,7 @@
  *
  * @author Andy Clement
  * @author Juergen Hoeller
+ * @author Sam Brannen
  * @since 3.0
  */
 public class MethodReference extends SpelNodeImpl {
@@ -72,6 +73,14 @@ public MethodReference(boolean nullSafe, String methodName, int startPos, int en
 	}
 
 
+	/**
+	 * Does this node represent a null-safe method reference?
+	 * @since 6.0.13
+	 */
+	public final boolean isNullSafe() {
+		return this.nullSafe;
+	}
+
 	/**
 	 * Get the name of the referenced method.
 	 */

spring-expression/src/test/java/org/springframework/expression/spel/ParsingTests.java

@@ -47,6 +47,7 @@ class Miscellaneous {
 		void compoundExpressions() {
 			parseCheck("property1.property2.methodOne()");
 			parseCheck("property1[0].property2['key'].methodOne()");
+			parseCheck("property1?.methodOne()?.property2?.methodTwo()");
 		}
 
 		@Test