Steps to reproduce:
* On macOS, run `input` example
* type `aaa|bbb` place caret on the place marked with |
* switch to `japanese romaji`
* press `ko`
* press left arrow

<img width="412" alt="image"
src="https://github.com/user-attachments/assets/d3c02e9b-98f9-420e-a3b7-681ba90829cd"
/>

You will get `aaa` duplicated with every arrow press.

According to [reference
implementation](https://developer.apple.com/library/archive/samplecode/TextInputView/Listings/FadingTextView_m.html#//apple_ref/doc/uid/DTS40008840-FadingTextView_m-DontLinkElementID_6)
we need to unmark text when we get empty line in `setMarkedText `