SANTA MONICA, Calif., Dec. 26, 1999 -- Perennial single guy Jerry Seinfeld finally threw in the towel Christmas Day, wedding public-relations executive Jessica Sklar in a small, hush-hush ceremony in New York City. It is the first marriage for Seinfeld, 45; the second (in 18 months) for Sklar, 28. The bride's initial husband-and-wife teamup (with theater scion Eric Nederlander) ended when she started dating Seinfeld weeks after she returned from her 1998 honeymoon. Seinfeld is, of course, best k