expander-row: Make the subtitle nullable

This matches the title, making the widget more consistent.
11 jobs for set-title in 15 minutes and 38 seconds (queued for 1 second)
Status Name Job ID Coverage
  Build
passed api-visibility #834092

00:00:32

passed build-debian-clang #834094

00:03:39

passed build-debian-gcc #834093

00:02:03

passed build-fedora-gcc #834095

00:05:58

passed build-flatpak #834096
flatpak

00:01:39

passed build-flatpak-glade #834097
flatpak

00:04:06

 
  Test+Docs
passed build-gtkdoc #834098

00:03:04

passed unit-test #834099

00:02:18

44.8%
 
  Package
passed package-debian-buster #834100

00:03:46

 
  Test Package
passed autopkgtest-debian-buster-package #834101

00:02:48

passed lintian-debian-buster-package #834102

00:00:54