background: Fix warning when XDG_PICTURES_DIR not defined

Make the code match the existing monitor to fall back to the home directory.

The error message was:
(gnome-control-center:22910): GLib-GIO-CRITICAL **: 16:42:17.915: g_file_new_for_path: assertion 'path != NULL' failed
11 jobs for background-no-pictures-dir in 4 minutes and 2 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Build
passed #127860
build

00:01:45

 
  Test
passed #127862
asan

00:02:16

passed #127861
test

00:00:38

 
  Manual
manual #127867
allowed to fail manual
armv8
manual #127863
allowed to fail manual
flatpak
manual #127866
allowed to fail manual
msan
manual #127868
allowed to fail manual
ppc64le
manual #127864
allowed to fail manual
tsan
manual #127865
allowed to fail manual
ubsan
 
  Review
manual #127869
allowed to fail manual
review
manual #127870
allowed to fail manual
stop_review