Goodlettsville-based Dollar General Corp. is cutting 255 jobs, the company announced today. "Substantially all" of the cuts are effective immediately, according to a news release which said the cuts are "part of a broader initiative aimed at proactively improving efficiencies and reducing expenses." Nearly half of the jobs being cut are vacant positions, the release says, and store-level positions have not been impacted. “Over the last several months, we have taken a hard look at our cost structure…