Fix: Changed the handling method of `Ime::Preedit(_, None)`
Fix: backspace fail after ime input
* Related #4358
* Related #4430
* Related #4436
* Related #4794
* Related #4896
* Closes#4908
Issues: backspace fail after ime input
* #4908 (Chinese)
Changed the handling method of `Ime::Preedit(_, None)`