`key` is an empty string in this case, causing `charCodeAt(0)` to return
`NaN`, which when passed to `Clutter.unicode_to_keysym` now generates an
error in gjs >= 1.67.3:
```
JS ERROR: Error: Argument wc: value is out of range for uint32
```
And the symbolic keys like Backspace, Enter and Caps Lock would have their
presses ignored.
Just skip the call to `charCodeAt` that will fail and allow
`Clutter.unicode_to_keysym` to return its usual error flag.
Fixes: https://bugs.launchpad.net/bugs/1918738
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1758>