Mk/bsd.port.mk: Allow DIST_SUBDIR to be defined, but empty

The rationale for this change is to allow ports to cancel out
DIST_SUBDIR that is set by some USES. For example, Uses/linux.mk has
DIST_SUBDIR?= rocky and we want to be able to opt-out of this in the port.

Approved by: bapt (portmgr)
Differential Revision: https://reviews.freebsd.org/D53028