Backspace key can remove format when delete the only character in a block

[JiuqingSong]

1. <div><span style="font-size: 24pt;">a</span></div><!--{"type":0,"isDarkMode":false,"start":[0,0,0,1],"end":[0,0,0,1]}-->
2. Backspace
3. Now it becomes <br><!--{"type":0,"isDarkMode":false,"start":[0],"end":[0]}-->
4. Type something else, format is lost

[JiuqingSong]

#1660