treewide: Container -> SizedBox.shrink()

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

lib/pages/chat/chat_app_bar_title.dart

@@ -17,7 +17,7 @@ class ChatAppBarTitle extends StatelessWidget {
   Widget build(BuildContext context) {
     final room = controller.room;
     if (room == null) {
-      return Container();
+      return const SizedBox.shrink();
     }
     if (controller.selectedEvents.isNotEmpty) {
       return Text(controller.selectedEvents.length.toString());

lib/pages/chat/chat_event_list.dart

@@ -91,7 +91,7 @@ class ChatEventList extends StatelessWidget {
                 ),
               );
             }
-            return Container();
+            return const SizedBox.shrink();
           }
 
           // The message at this index:
@@ -127,7 +127,7 @@ class ChatEventList extends StatelessWidget {
                         ? controller.timeline!.events[i]
                         : null,
                   )
-                : Container(),
+                : const SizedBox.shrink(),
           );
         },
         childCount: controller.timeline!.events.length + 2,

lib/pages/chat/chat_input_row.dart

@@ -23,7 +23,7 @@ class ChatInputRow extends StatelessWidget {
   Widget build(BuildContext context) {
     if (controller.showEmojiPicker &&
         controller.emojiPickerType == EmojiPickerType.reaction) {
-      return Container();
+      return const SizedBox.shrink();
     }
     return Row(
       crossAxisAlignment: CrossAxisAlignment.end,
@@ -72,7 +72,7 @@ class ChatInputRow extends StatelessWidget {
                             ),
                           ),
                         )
-                  : Container(),
+                  : const SizedBox.shrink(),
             ]
           : <Widget>[
               KeyBoardShortcuts(

lib/pages/chat/events/message.dart

@@ -54,7 +54,7 @@ class Message extends StatelessWidget {
       EventTypes.CallInvite
     }.contains(event.type)) {
       if (event.type.startsWith('m.call.')) {
-        return Container();
+        return const SizedBox.shrink();
       }
       return StateMessage(event);
     }

lib/pages/chat/input_bar.dart

@@ -313,7 +313,7 @@ class InputBar extends StatelessWidget {
         ),
       );
     }
-    return Container();
+    return const SizedBox.shrink();
   }
 
   void insertSuggestion(_, Map<String, String?> suggestion) {
@@ -458,11 +458,11 @@ class InputBar extends StatelessWidget {
               buildSuggestion(c, s, Matrix.of(context).client),
           onSuggestionSelected: (Map<String, String?> suggestion) =>
               insertSuggestion(context, suggestion),
-          errorBuilder: (BuildContext context, Object? error) => Container(),
-          loadingBuilder: (BuildContext context) => Container(),
+          errorBuilder: (BuildContext context, Object? error) => const SizedBox.shrink(),
+          loadingBuilder: (BuildContext context) => const SizedBox.shrink(),
           // fix loading briefly flickering a dark box
           noItemsFoundBuilder: (BuildContext context) =>
-              Container(), // fix loading briefly showing no suggestions
+              const SizedBox.shrink(), // fix loading briefly showing no suggestions
         ),
       ),
     );

lib/pages/chat/pinned_events.dart

@@ -49,7 +49,7 @@ class PinnedEvents extends StatelessWidget {
     final pinnedEventIds = controller.room!.pinnedEventIds;
 
     if (pinnedEventIds.isEmpty) {
-      return Container();
+      return const SizedBox.shrink();
     }
     final completers = pinnedEventIds.map<Completer<Event?>>((e) {
       final completer = Completer<Event?>();

lib/pages/chat/reactions_picker.dart

@@ -15,7 +15,7 @@ class ReactionsPicker extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
-    if (controller.showEmojiPicker) return Container();
+    if (controller.showEmojiPicker) return const SizedBox.shrink();
     final display = controller.editEvent == null &&
         controller.replyEvent == null &&
         controller.room!.canSendDefaultMessages &&
@@ -29,7 +29,7 @@ class ReactionsPicker extends StatelessWidget {
         child: Builder(
           builder: (context) {
             if (!display) {
-              return Container();
+              return const SizedBox.shrink();
             }
             final proposals = proposeEmojis(
               controller.selectedEvents.first.plaintextBody,

lib/pages/chat/reply_display.dart

@@ -58,7 +58,7 @@ class _EditContent extends StatelessWidget {
   Widget build(BuildContext context) {
     final event = this.event;
     if (event == null) {
-      return Container();
+      return const SizedBox.shrink();
     }
     return Row(
       children: <Widget>[

lib/pages/chat/sticker_picker_dialog.dart

@@ -39,7 +39,7 @@ class StickerPickerDialogState extends State<StickerPickerDialog> {
       final imageKeys =
           filteredImagePackImageEntried.map((e) => e.key).toList();
       if (imageKeys.isEmpty) {
-        return Container();
+        return const SizedBox.shrink();
       }
       final packName = pack.pack.displayName ?? packSlugs[packIndex];
       return Column(

lib/pages/chat/tombstone_display.dart

@@ -12,7 +12,7 @@ class TombstoneDisplay extends StatelessWidget {
   @override
   Widget build(BuildContext context) {
     if (controller.room!.getState(EventTypes.RoomTombstone) == null) {
-      return Container();
+      return const SizedBox.shrink();
     }
     return SizedBox(
       height: 72,