Fixes https://github.com/flutter/flutter/issues/130685

This fixes an issue where a GridView/SliverGrid with a SliverChildBuilderDelegate would still reflect a max scroll extent of infinity after returning null from the builder. This change incorporates the same way we handle this case in SliverList:

https://github.com/flutter/flutter/blob/6d87a23b3722e4fb2e2db993322d5be4adf04fb5/packages/flutter/lib/src/rendering/sliver_list.dart#L305-L307