The Boeing Co. was first. Then Lockheed Martin Corp. And Wednesday, Northrop Grumman Corp.’s chief financial officer seemed to be prepping analysts for a potential change. So might the Falls Church company be next to freeze its pension plan? The timing wouldn’t be random. Beyond the fact that defense companies carry a lot of liability, due largely to the fact that they’ve been around a while, the Society of Actuaries was expected to issue new proposed guidance for mortality assumptions.