If the null cipher is used, allow only empty password for LUKS.

The cipher_null is no-encryption, it can be used for testing
or temporarily when encrypting device (cryptsetup-reencrypt).

Accepting only empty password prevents situation when you replace
a LUKS header on an unlocking device with the faked header using
null cipher (and the same UUID).
Here a system could think that the device was properly unlocked
(with any entered password) and will try to use this unencrypted
partition instead.
(IOW it prevents situation when attacker intentionaly forces
an user to boot into dirrerent system just by LUKS header manipulation.)

Properly configured systems should have an additional integrity protection
in place here (LUKS here provides only confidentiality) but it is better
to not not allow this situation in the first place.
(Despite the fact that once you allow physical tampering of your system
it cannot be properly secured anymore.)