expander-row: Make the subtitle nullable

This matches the title, making the widget more consistent.
11 jobs for set-title in 18 minutes and 33 seconds (queued for 1 second)
Status Name Job ID Coverage
  Build
passed api-visibility #831034

00:01:25

passed build-debian-clang #831036

00:06:45

passed build-debian-gcc #831035

00:01:56

passed build-fedora-gcc #831037

00:03:36

passed build-flatpak #831038
flatpak

00:01:47

passed build-flatpak-glade #831039
flatpak

00:04:03

 
  Test+Docs
passed build-gtkdoc #831040

00:03:27

passed unit-test #831041

00:03:31

44.9%
 
  Package
passed package-debian-buster #831042

00:05:26

 
  Test Package
passed autopkgtest-debian-buster-package #831043

00:02:48

passed lintian-debian-buster-package #831044

00:01:13