Fix: Resolved issue with broken IME composing rect in Windows desktop app (singerdmx#2282)

- Fixed issue where the composing rect would break during IME conversion.
- When the composing region is unknown, it now avoids calling setComposingRect on TextInputConnection.

Author: agata

lib/src/editor/raw_editor/raw_editor_state_text_input_client_mixin.dart

@@ -117,10 +117,12 @@ mixin RawEditorStateTextInputClientMixin on EditorState
         textEditingValue.composing;
     if (hasConnection) {
       assert(mounted);
-      final offset = composingRange.isValid ? composingRange.start : 0;
-      final composingRect =
-          renderEditor.getLocalRectForCaret(TextPosition(offset: offset));
-      _textInputConnection!.setComposingRect(composingRect);
+      if (composingRange.isValid) {
+        final offset = composingRange.start;
+        final composingRect =
+            renderEditor.getLocalRectForCaret(TextPosition(offset: offset));
+        _textInputConnection!.setComposingRect(composingRect);
+      }
       SchedulerBinding.instance
           .addPostFrameCallback((_) => _updateComposingRectIfNeeded());
     }