Ascii with empty canadidate

It's really hard to test ibus 1.5 on Debian. I end up did a `s/IBus\.KEY_/IBus./g` in engine/table.py. And tested it with ibus 1.4. Well, ibus and ibus-table ran. That's all I can say.

Ibus 1.4 -> 1.5 upgrade really screwed up for everyone.