treewide: Container -> SizedBox.shrink()

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

lib/widgets/chat_settings_popup_menu.dart

@@ -115,7 +115,7 @@ class ChatSettingsPopupMenuState extends State<ChatSettingsPopupMenu> {
           },
           helpLabel: L10n.of(context)!.chatDetails,
           onKeysPressed: _showChatDetails,
-          child: Container(),
+          child: const SizedBox.shrink(),
         ),
         KeyBoardShortcuts(
           keysToPress: {
@@ -124,7 +124,7 @@ class ChatSettingsPopupMenuState extends State<ChatSettingsPopupMenu> {
           },
           helpLabel: L10n.of(context)!.matrixWidgets,
           onKeysPressed: _showWidgets,
-          child: Container(),
+          child: const SizedBox.shrink(),
         ),
         PopupMenuButton(
           onSelected: (String choice) async {

lib/widgets/mxc_image.dart

@@ -163,7 +163,7 @@ class _MxcImageState extends State<MxcImage> {
           data == null ? CrossFadeState.showFirst : CrossFadeState.showSecond,
       firstChild: placeholder(context),
       secondChild: data == null || data.isEmpty
-          ? Container()
+          ? const SizedBox.shrink()
           : Image.memory(
               data,
               width: widget.width,