Fix issue when a key name could be unknown

[andydotxyz]

Fixes #89

[xaionaro]

Before

xaionaro@void:~$ fyneterm

Switch the layout to Russian and press CTRL + '=':

panic: runtime error: index out of range [0] with length 0

goroutine 12 [running]:
github.com/fyne-io/terminal.(*Terminal).TypedShortcut(0xc00027e400, {0xd98a60?, 0xc0066d0eb8?})
	/home/xaionaro/.gvm/pkgsets/go1.22.1/global/pkg/mod/github.com/fyne-io/[email protected]/input.go:170 +0x23b
fyne.io/fyne/v2/internal/driver/glfw.(*window).triggersShortcut.func1()
	/home/xaionaro/.gvm/pkgsets/go1.22.1/global/pkg/mod/fyne.io/fyne/[email protected]/internal/driver/glfw/window.go:912 +0x24
fyne.io/fyne/v2/internal/driver/common.(*Window).RunEventQueue(...)
	/home/xaionaro/.gvm/pkgsets/go1.22.1/global/pkg/mod/fyne.io/fyne/[email protected]/internal/driver/common/window.go:35
created by fyne.io/fyne/v2/internal/driver/glfw.(*gLDriver).createWindow.func1 in goroutine 1
	/home/xaionaro/.gvm/pkgsets/go1.22.1/global/pkg/mod/fyne.io/fyne/[email protected]/internal/driver/glfw/window.go:997 +0x136

After

xaionaro@void:~$ go install github.com/fyne-io/terminal/cmd/fyneterm@6d9024789cffec0be7ca224cfaab4f6a773f3388
go: downloading github.com/fyne-io/terminal v0.0.0-20240711112249-6d9024789cff

xaionaro@void:~$ fyneterm

Switch the layout to Russian and press CTRL + '=': nothing happens.

[andydotxyz]

Switch the layout to Russian and press CTRL + '=': nothing happens.

What should Ctrl+= perform?

[xaionaro]

Switch the layout to Russian and press CTRL + '=': nothing happens.

What should Ctrl+= perform?

Well, I hoped to increase font size. But here I'm just saying that the bugfix helped and now it no longer panics (thus "nothing happens") :)

[andydotxyz]

Increasing font size is Ctrl+"+" (ctrl + shift + "=" on my keyboard)