Let ibus-ui-gtk3 not to show zero-lenght preedit text.

preedit text is shown by ibus-ui-gtk3 on X11 applications
and do not have to show it likes GTK+ when the lenght is zero.

BUG=http://code.google.com/p/ibus/issues/detail?id=1709
TEST=ibus-ui-gtk3 with ibus-pinyin and x11 applications

Review URL: https://codereview.appspot.com/93150043