Fix crash on activating a text area without a real keyboard

It will cause on following situation:

* Enable weston-simple-im
  * It's also reproducible with IBus or Fcitx5
* Connect a mouse or a touch panel
* Don't connect keyboad
* Launch weston-editor and touch the text area

It doesn't reproduce with weston-keyboard because it doesn't use
grab_keyboard of input-method-context-v1.

Signed-off-by: Takuro Ashie <ashie@clear-code.com>