cairo-util: Don't set title string to Pango layout if the title is NULL

If buttons list isn't empty and title is NULL, SEGV is occured in
pango_layout_set_text(). This patch fixes this problem.

Signed-off-by: Tomohito Esaki <etom@igel.co.jp>
2 jobs for wm-fix-title-layout in 1 minute and 10 seconds (queued for 50 seconds)
Status Job ID Name Coverage
  Container Prep
passed #210712
container_prep

00:00:37

 
  Build
passed #210713
build-native-meson

00:00:33