print_ast: use wrap util function in more places

Replicates graphql/graphql-js@d0eae6f

Author: Cito

src/graphql/language/printer.py

@@ -278,7 +278,7 @@ def leave_union_type_definition(node: PrintedNode, *_args: Any) -> str:
                 "union",
                 node.name,
                 join(node.directives, " "),
-                "= " + join(node.types, " | ") if node.types else "",
+                wrap("= ", join(node.types, " | ")),
             ),
             " ",
         )
@@ -360,7 +360,7 @@ def leave_union_type_extension(node: PrintedNode, *_args: Any) -> str:
                 "extend union",
                 node.name,
                 join(node.directives, " "),
-                "= " + join(node.types, " | ") if node.types else "",
+                wrap("= ", join(node.types, " | ")),
             ),
             " ",
         )