Create a method that upserts Contact records using DML operations.

Parameters:

• contacts (List<Contact>): List of Contact records to upsert

Returns:

• Integer: The number of contacts processed, or 0 if none

Requirements:

• Use DML upsert operation
• Handle null or empty contacts list by returning 0
• Use Email field as external ID for upsert
• Handle DML exceptions gracefully

Example:

List<Contact> contacts = new List<Contact>{
    new Contact(FirstName = 'John', LastName = 'Doe', Email = 'john@example.com')
};
Integer processed = upsertContacts(contacts);
// Returns: 1