On a night that NBC use to call Must-See-TV night when it dominated the ratings on Thursdays, there really was only one must-see telecast to watch -- the seventh and deciding game of the NBA Finals in which the Los Angeles Lakers defeated the Boston Celtics. The Game averaged a whopping 26 percent of the viewing audience, peaking in the final half hour with 36 percent, according to preliminary overnight Nielsen figures. That percentage translated to 18.86 million viewers, more than the other three networks combined. (23.40 million at 10 30 p.m.) In fact, the numbers are likely to increase significantly since they do not include West Coast results. (Streets were virtually deserted in Los Angeles during the last hour of the game.) Last year, the fifth and final game of the championship series between the Lakers and the Orlando Magic drew 14 million viewers.