Yast installer does not allow to select random password for swap partitions.
In previous leap editions leaving the encryption password blank for a swap
partition means to select random password for swap partition and. This
translate into something similar inside /etc/crypttab:

cr_swap         /dev/disk/by-id/md-uuid-070... /dev/urandom swap

cr_swap is reinitialized with a new random password at every reboot. 

Yast installer of leap 15 beta 187.1 does not allow to leave the password blank
and does not exhibits any other way to specify random password for swap.