If only we could hear Norma Desmond rise up and sing: Don’t cha wish your girflriend, that little tart Betty Schaefer, was hot like me? That does not happen in Nicole Scherzinger ‘s current run of shows, which included a diverting stop Thursday night at Walt Disney Concert Hall in downtown L.A. The show tunes stayed in their corner, and the Pussycat Dolls oldies stayed in theirs. It was a show with a slightly split personality, with musical theater numbers dominating the first three-quarters