It sounds like you completed PreTrial Intervention, and after completing the program, the State dropped the case. If this is correct, you did not receive a withhold of adjudication, nor were you convicted of a crime.
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.