ODD COUPLE: False Hollywood rumor of the week? That Debi Mazar (L.A. Law) and Paul Reubens (a.k.a. Pee-wee Herman) are engaged. It wasn't long after the supermarket tabloid The Globe printed the story that Mazar's agent was getting calls from more respected news outlets that had taken the fictitious story at face value. Reubens, a longtime friend of the actress, decided to play along with the tale by presenting Mazar with a four-carat diamond ring. ''Paul and I are crazy about each other,'' admits Mazar, ''and, yes, he gave me a ring. As for it being an engagement, do you believe everything you read in The Globe?''