[refactoring] update ast structure of "Nullable"

internal/php7/php7.go

@@ -4343,13 +4343,13 @@ yydefault:
 		yyDollar = yyS[yypt-2 : yypt+1]
 // line internal/php7/php7.y:1950
 		{
-			yyVAL.node = &ast.Nullable{ast.Node{}, yyDollar[2].node}
-
-			// save position
-			yyVAL.node.GetNode().Position = position.NewTokenNodePosition(yyDollar[1].token, yyDollar[2].node)
-
-			// save comments
-			yylex.(*Parser).setFreeFloating(yyVAL.node, token.Start, yyDollar[1].token.SkippedTokens)
+			yyVAL.node = &ast.Nullable{
+				Node: ast.Node{
+					Position: position.NewTokenNodePosition(yyDollar[1].token, yyDollar[2].node),
+				},
+				QuestionTkn: yyDollar[1].token,
+				Expr:        yyDollar[2].node,
+			}
 		}
 	case 223:
 		yyDollar = yyS[yypt-1 : yypt+1]

internal/php7/php7.y

@@ -1948,13 +1948,13 @@ type_expr:
             }
     |   '?' type
             {
-                $$ = &ast.Nullable{ast.Node{}, $2}
-
-                // save position
-                $$.GetNode().Position = position.NewTokenNodePosition($1, $2)
-
-                // save comments
-                yylex.(*Parser).setFreeFloating($$, token.Start, $1.SkippedTokens)
+                $$ = &ast.Nullable{
+                    Node: ast.Node{
+                        Position: position.NewTokenNodePosition($1, $2),
+                    },
+                    QuestionTkn: $1,
+                    Expr:        $2,
+                }
             }
 ;
 

pkg/ast/node.go

@@ -33,6 +33,7 @@ func (n *Root) Accept(v NodeVisitor) {
 // Nullable node
 type Nullable struct {
 	Node
+	QuestionTkn *token.Token
 	Expr        Vertex
 }