Create a method that upserts Account records using DML operations.

Parameters:

• accounts (List<Account>): List of Account records to upsert

Returns:

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

Requirements:

• Use DML upsert operation
• Handle null or empty accounts list by returning 0
• Handle DML exceptions gracefully
• Return the count of successfully processed records

Example:

List<Account> accounts = new List<Account>{
    new Account(Name = 'Acme Corp', AccountNumber = 'EXT001')
};
Integer processed = upsertAccountsWithExternalId(accounts);
// Returns: 1