treewide: Container -> SizedBox.shrink()

This helps performance without any user-facing changes,
since SizedBox is constant while Container isn't

lib/pages/chat_details/chat_details_view.dart

@@ -409,7 +409,7 @@ class ChatDetailsView extends StatelessWidget {
                                   ),
                                   onTap: () => VRouter.of(context).to('invite'),
                                 )
-                              : Container(),
+                              : const SizedBox.shrink(),
                         ],
                       )
                     : i < controller.members!.length + 1

lib/pages/chat_details/participant_list_item.dart

@@ -62,7 +62,7 @@ class ParticipantListItem extends StatelessWidget {
                 ),
               ),
             membershipBatch[user.membership]!.isEmpty
-                ? Container()
+                ? const SizedBox.shrink()
                 : Container(
                     padding: const EdgeInsets.all(4),
                     margin: const EdgeInsets.symmetric(horizontal: 8),