bsdinstall zfsboot: Don't override ZFSBOOT_FORCE_4K_SECTORS if it is null.

Only set a default value of 1 if the shell variable is unset. This allows
installer scripts to disable the variable.

PR: 274513
Reported by: Albin "a12l" Otterhäll <bugs.freebsd.org@a12l.xyz>
Differential Revision: https://reviews.freebsd.org/D42319