Fix empty keystroke with simulated IME (#19414)

Closes #19181

When the keystroke was empty ("") the `ime_key` was converted from
`None` to `Some("")` when `with_simulated_ime` was called. That was
leading to not intentional behavior when an empty keystroke was combined
with `shift-up` in a keybinding `["workspace::SendKeystrokes", "shift-up
"]`.

By adding a `key.is_empty()` we make sure the `ime_key` keeps as `None`.

This was manually tested. 

Release Notes:

- Fixed empty keystroke with simulated ime

Signed-off-by: Bruno Calza <brunoangelicalza@gmail.com>