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