fix: unexpect behaviour when pressing enter key

frontend/app_flowy/packages/flowy_editor/lib/src/service/internal_key_event_handlers/enter_without_shift_in_text_node_handler.dart

@@ -67,7 +67,7 @@ FlowyKeyEventHandler enterWithoutShiftInTextNodesHandler =
   // If selection is collapsed and position.start.offset == 0,
   //  insert a empty text node before.
   if (selection.isCollapsed && selection.start.offset == 0) {
-    if (textNode.toRawString().isEmpty) {
+    if (textNode.toRawString().isEmpty && textNode.subtype != null) {
       final afterSelection = Selection.collapsed(
         Position(path: textNode.path, offset: 0),
       );