Retain null-safe syntax in AST representation of SpEL indexers

Prior to this commit, SpEL's CompoundExpression omitted the null-safe
syntax in AST string representations of indexing operations.

To address this, this commit implements isNullSafe() in Indexer.

See gh-29847

Author: sbrannen

Diff for: spring-expression/src/main/java/org/springframework/expression/spel/ast/Indexer.java

@@ -127,6 +127,15 @@ public Indexer(boolean nullSafe, int startPos, int endPos, SpelNodeImpl indexExp
 	}
 
 
+	/**
+	 * Does this node represent a null-safe index operation?
+	 * @since 6.2
+	 */
+	@Override
+	public final boolean isNullSafe() {
+		return this.nullSafe;
+	}
+
 	@Override
 	public TypedValue getValueInternal(ExpressionState state) throws EvaluationException {
 		return getValueRef(state).getValue();

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

@@ -60,8 +60,8 @@ void compoundExpressions() {
 			parseCheck("property1?.property2?.methodOne()");
 			parseCheck("property1?.methodOne('enigma')?.methodTwo(42)");
 			parseCheck("property1?.methodOne()?.property2?.methodTwo()");
-			parseCheck("property1[0]?.property2['key']?.methodTwo()");
-			parseCheck("property1[0][1]?.property2['key'][42]?.methodTwo()");
+			parseCheck("property1?.[0]?.property2?.['key']?.methodTwo()");
+			parseCheck("property1?.[0]?.[1]?.property2?.['key']?.[42]?.methodTwo()");
 		}
 
 		@Test