mail/opendkim: Fix errors when no value is set for socket in RC script

Fix errors

PR: ports/222122
Submitted by: Matthias Pfaller <matthias.pfaller@familie-pfaller.de>

			(Fix)

While here, various style fixes; no need for the "x$var" != "x" idiom
as that is confusing to follow.

Approved by: maintainer timeout (3 years)