cryptsetup: don't add unit dependency on /dev/null devices when it is listed as password file

As special magic, don't create device dependencies for /dev/null. Of
course, there might be similar devices we might want to include, but
given that none of them really make sense to specify as password source
there's really no point in checking for anything else here.

https://bugs.freedesktop.org/show_bug.cgi?id=75816