Composer: Add check for empty input to prevent errors on delete action

[gjf7]

This PR fixes #363.

Also fixed two issues shown in the video below:

1. Unable to delete when only one emoji is left.
2. Deleted the wrong emoji when there are multiple emojis.

https://github.com/Ajaxy/telegram-tt/assets/77763617/b93839b3-cb65-4754-97c1-7a6e4fbbfeeb