I had a misdemeanor charged resolved this way, but it isn't clear how I should answer a question on an application form that says "Have you ever committed or been convicted of a crime (felony or misdemeanor0, had an adjudication witheld, received a suspended sentence or entered a plea of Nolo Contendere?"
Despite the fact that the case was dropped, you might want to consider getting it sealed and expunged. Some of the common applications have very poorly worded questions now. Asking someone if they have ever "committed" a crime is much different than asking if they ever pled or had adjudication withheld. Some PTI programs result in the case being dropped only if you admit the crime through a statement or letter of apology.
In any case, you have not been convicted, had adjudication withheld, received a suspended sentence or entered a plea of nolo contendere if you completed PTI and received a Nolle Prosequi. Good luck.