A Nolle Prosequi is simply the prosecution dropping the charges. Your record will continue to show an arrest for petit theft until you have your record expunged. I'm assuming you are eligible for expungement because you were offered diversion and the charges have been dropped.
However, regardless of whether you seek expungement of your record, you do not have a conviction on your record. This distinction is important for job applications as some potential employers only care about convictions and not arrests. If you get your record expunged you can also deny that you were even arrested.
You should seek the advice of an attorney to see whether you qualify for expungement and then seek it if you are eligible.
"Nolle prosequi" is a Latin phrase that literally means "no prosecution." In many Florida counties, diversion programs are available, and the case is dropped with a nolle prosequi after successful completion of the program.
Your record generally will indicate the charge and that a nolle prosequi was entered. If you have no other criminal history, you can apply to the Florida Department of Law Enforcement (FDLE) to have your record sealed or expunged, effectively removing it from your record. You might want to consult a lawyer to help you make this decision.