Mk/bsd.port.mk: Don't pass up an empty flavor.

The classic way to handle flavors is to set:

FLAVORS= foo bar
FLAVOR?= ${FLAVORS:[0]}

And in that case, FLAVOR is only set if the variable is not defined. If
you pass an empty flavor using FLAVOR= then it remains empty after
that line. It can leads to some ports with complicated logic to assume
the wrong flavor is set.

PR: 256301
Reported by: avg
Differential Revision: https://reviews.freebsd.org/D30579