There had been other gay characters on TV series before public administrative assistant John Irvin on ABC's "NYPD Blue", but perhaps none to whom the audience had so quickly and easily warmed. With his sweet smile and peppy attitude, Bill Brochtrup, the openly gay blond with boy-next-door good looks, won the role in 1994 after playing a homosexual model with AIDS in the L.A. stage production of "The Raft of the Medusa". What followed during two seasons of recurring appearances was a chance to see not just Brochtrup's fine acting, but the character of Det. Sipowicz (Dennis Franz) finding he