Petty theft is considered a crime of moral turpitude but since you were able to expunge your case this would indicate to me that you were never convicted of the crime.
From your narrative, it appears that you entered a pretrial diversion program and ultimately received a dismissal of your case.
If you were a juvenile and have since expunged your record, then your arrest no longer exists.
You may legally deny ever having being arrested. (Unless, for instance, you were applying for a position in law-enforcement, as a teacher, etc and you were specifically asked if you ever had expunge a record)
Best if luck.
The above is not intended as legal advice. The response does not constitute the creation of an attorney client relationship as this forum does not provide for a confidential communication.