There are times when the program requires an entry other than "0", even when "0" is the appropriate value for that field. Using "$1" in these cases will not affect on any tax calculation or your ability to efile, but it will "tell" the program that you have made an entry required to support a deduction or credit.
In this case, entering "$1" for both the beginning year loan balance and the year-end loan balance lets the program know that you didn't forget to report at least some level of debt that would account for an interest expense deduction.
From an economic perspective, it accurately reflects that you owed the same amount (virtually nothing) at both the beginning and the end of the year.
