Editorial: Correct return type of GetDateTimeFormat

Some of the DateTimeFormat internal slots may contain null instead of a
DateTime Format Record, if no relevant fields were given. That needs to
be reflected in the return type of this operation.

spec/intl.html

@@ -820,7 +820,7 @@ <h1>
             _required_: ~date~, ~time~, ~year-month~, ~month-day~, or ~any~,
             _defaults_: ~date~, ~time~, ~year-month~, ~month-day~, ~zoned-date-time~, or ~all~,
             _inherit_: ~all~ or ~relevant~,
-          ): a DateTime Format Record
+          ): a DateTime Format Record or *null*
         </h1>
         <dl class="header">
         </dl>