Don't warn if DISPLAY is not set
Don't warn if DISPLAY is not set

This is normal under Wayland, and not worth warning about.
The warnings disrupt unit tests in GNOME continuous, which
treat warnings as fatal.

BUG=https://github.com/ibus/ibus/pull/1844
R=shawn.p.huang@gmail.com

Review URL: https://codereview.appspot.com/289430043

Patch from Matthias Clasen <mclasen@redhat.com>.

fujiwarat/ibus,fujiwarat/ibus,fujiwarat/ibus,Keruspe/ibus,ibus/ibus,ibus/ibus,Keruspe/ibus,ibus/ibus,ibus/ibus,Keruspe/ibus,Keruspe/ibus,fujiwarat/ibus