Let me be clear: you have nothing to report. You had an arrest and then the case was dismissed. You do not have a conviction of any crime. There is nothing, repeat, nothing that you need to report to AMCAS. The record is sealed and you can reasonably make the decision that even if asked about an arrest, you could leave this off as it violate the spirit and intent of sealing the case. Alternatively, you could report arrest for petty theft but case was dismissed. Again, there is nothing to report here
Misdemeanor
You must indicate whether you ever been convicted of, or pleaded guilty or no contest to, a Misdemeanor crime, excluding 1) any offense for which you were adjudicated as a juvenile, 2) any convictions which have been expunged or sealed by a court, or 3) any misdemeanor convictions for which any probation has been completed and the case dismissed by the court (in states where applicable)