Is it possible to only update the contact's properties which don't exist in the Hubspot? For e.g., if I have a property like city which I need to update only if a contact doesn't have any city assigned to it. Is it possible to get the property of the user that already exists using tracking code API?
It's not possible to update only a contact's unknown property values without first checking their values with the Contacts API.
The Tracking Code API is effectively a one-way street into HubSpot, so you unfortunately won't be able to check a visitor's
city value. You will just be able to update it.