ukify: don't panic when prepending to an undefined list

ukify: don't panic when prepending to an undefined list

Handle the case when all the arguments are passed in through a
configuration file:

$ cat ukify.conf
[UKI]
Linux = /boot/vmlinuz-linux
Initrd = /boot/initramfs-linux.img

Before:
$ src/ukify/ukify.py --config ukify.conf build
Traceback (most recent call last):
  File "/root/systemd/src/ukify/ukify.py", line 1604, in <module>
    main()
  File "/root/systemd/src/ukify/ukify.py", line 1590, in main
    opts = parse_args()
           ^^^^^^^^^^^^
  File "/root/systemd/src/ukify/ukify.py", line 1584, in parse_args
    apply_config(opts)
  File "/root/systemd/src/ukify/ukify.py", line 1431, in apply_config
    item.apply_config(namespace, section_name, group, key, value)
  File "/root/systemd/src/ukify/ukify.py", line 1123, in apply_config
    self.config_push(namespace, group, dest, value)
  File "/root/systemd/src/ukify/ukify.py", line 1019, in config_list_prepend
    setattr(namespace, dest, value + old)
                             ~~~~~~^~~~~
TypeError: can only concatenate list (not "NoneType") to list

After:
$ src/ukify/ukify.py --config ukify.conf build
Kernel version not specified, starting autodetection 😖.
Found uname version: 6.4.7-arch1-3
Wrote unsigned vmlinuz-linux.unsigned.efi

Resolves: #28688