sideload-repo-systemd: Set 'nullglob' option

Previously, if there were no existing symlinks, this script would fail:

    wjt@camille:~$ LANG=en_GB.utf8 bash -x /usr/libexec/flatpak-create-sideload-symlinks.sh /run/media/wjt
    + test 1 -eq 1
    + test -d /run/media/wjt
    + for f in "$1"/*
    + test -d '/run/media/wjt/*'
    + continue
    + for f in /run/flatpak/sideload-repos/automount-*
    + test -e '/run/flatpak/sideload-repos/automount-*'
    + rm '/run/flatpak/sideload-repos/automount-*'
    rm: cannot remove '/run/flatpak/sideload-repos/automount-*': No such file or directory

This is due to the surprising POSIX shell behaviour that a glob that
matches no files expands to itself, rather than to the empty list.

http:https://mywiki.wooledge.org/BashFAQ/004

The POSIX solution is to add 'test -L $f' inside the loop to check if
the variable actually exists.  The first loop in this script uses this
technique: it has a 'test -d "$f"', seen in the trace above.

Bash implements a 'nullglob' feature which gives globs the behaviour you
might hope for. Set it in this script.

sideload-repos-systemd/flatpak-create-sideload-symlinks.sh

@@ -1,6 +1,7 @@
 #!/bin/bash
 
 # This script is intended to be run by flatpak-sideload-usb-repo.service
+shopt -s nullglob
 
 if ! test $# -eq 1 || ! test -d "$1"; then
  echo "Error: first argument must be a directory"