PBS' continuing series "Pioneers in Television" features an episode on sci-fi programming Tuesday night that includes an extensive interview with "Star Trek" star Nichelle Nichols. The timing of the telecast (Ch. 13, 8 p.m.) is accidental, but is significant because it falls a day after today's Martin Luther King holiday. Had it not been for King, Nichols' career may have been different. After a year with"Star Trek" as communications officer Lieutenant Uhura, she turned in her resignation. But at an NAACP event that weekend, she ran into King. "One of the promoters came up and said someone wanted to meet...