cryptsetup-generator: Don't mistake NULL input as OOM (#7688)

cryptsetup-generator: Don't mistake NULL input as OOM (#7688)

Since systemd v236, several Arch users complained that
systemd-cryptsetup-generator exits with an OOM error and that it
prevents the boot from continuing.

Investigating the diff of cryptsetup-generator between v235 and v236 I
noticed that create_disk allowed for the `password` and `filtered`
variables to be NULL (they're handled with `strempty()`) but not their
`*_escaped` versions, and returned OOM errors in those cases.

Fix this by checking that the input string is non-NULL before deciding
that `specifier_escape` had an OOM error.

I could not test this fix myself, but some users have reported success.

Downstream bug: https://bugs.archlinux.org/task/56733