Can a bull tiptoe through a tulip bed? Last week, Merrill Lynch's two money market funds, controlling $32 billion, shifted some $2.1 billion out of the commercial paper market and into government securities.According to John Hewitt, senior vice-president of Merrill Lynch Asset Management, the shift had nothing to do with questions about the quality of credit in the commercial paper market. Rather, he says, the funds, which represent 6 percent of funds' assets, were moved because the government sector had longer maturities. He points out that the average maturity of the funds has increased from 30 days to 45 days over the past six weeks. ''We thought the easiest way to lengthen the portfolio was to buy Treasury issues, since they were in the size and maturity we wanted.''What does Mr. Hewitt think about possible problems in the commercial paper market? He says the borrowers Merrill Lynch is lending to are ''some high-quality names, some of whom have not used the commercial paper market in the past.'' He adds, ''It's my impression Wall Street is well aware of the problems and is screening commercial paper customers carefully.''