There was a tense moment at the Boston Tug Muster, held this year on the last day of May. At 10 A.M. sharp, the official opening moment of this 19th annual event, there were no tugs at the rallying point, Pier 4, Charlestown. At 10:05, still no tugs. By 10:10, only Innovator, possibly the shortest tug in town, had cruised by. It passed along the pier as if looking for old friends, and finding none, performed its trademark about-face and seemed to be departing. Maybe the gents aboard had got the date wrong? Last year's Muster, after all, was in August. On the pier itself, among Muster officials, a nasty question was starting to form: What if you gave a Muster, and nobody came?