Take a List of Contact records and upsert them using the Email field as the unique identifier. Each contact has FirstName, LastName, and Email populated. If a contact with the same email already exists, update their FirstName and LastName. Return the list of upserted contacts with their IDs. Return empty list if input is null or empty.

Examples:

Input: contacts = [Contact(FirstName='John', LastName='Doe', Email='john@example.com'), Contact(FirstName='Jane', LastName='Smith', Email='jane@example.com')]
Output: List of 2 Contact records with IDs populated
Explanation: Inserts 2 new Contact records using Email field as unique identifier and returns them with IDs

Input: contacts = [Contact(FirstName='John Updated', LastName='Doe Updated', Email='john@example.com')]
Output: List of 1 Contact record with existing ID
Explanation: Updates existing Contact with matching Email address and returns it with ID

Input: contacts = []
Output: Empty list
Explanation: Returns empty list for empty input