A tractor-trailer driver has been cleared in a collision that left a bicyclist dead last October in Cambridge’s Porter Square after investigators determined the rider veered into the truck’s blind spot without signaling, authorities said.Bernard Lavins, 60, of Lexington, a pharmaceutical company executive, was killed Oct. 5 when his bike veered into the middle lane of traffic on Massachusetts Avenue near Porter Square, according to Middlesex District Attorney Marian T.