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/test/java/org/springframework/expression/spel/ParsingTests.java

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