Signed-off-by: erick-alcachofa <erick@artichoke.dev>
This commit refactors several AST nodes to use `Optional<T>` for fields
that are not always present. This includes `attribute` in
`ReflectionExpression`, `elseBranch` in `IfStatement` and
`WhileStatement`, `defaultCase` in `MatchStatement` and
`SwitchStatement`, and `preLoop` and `postLoop` in `CForStatement`. This
change improves the robustness and clarity of the AST by explicitly
modeling optionality.