If the form reports a zero outstanding balance in box 2 there would therefore be zero interest paid. You need a principal to calculate interest. The absolute best way to handle is for the lender to fix the 1098 and put an outstanding amount in box 2. Below is extracted from page 2 of the Form 1098 instructions.
Lines of credit and credit card obligations. Interest (other than points) received on any mortgage that is in the form of a line of credit or credit card obligation is reportable regardless of how you classified the obligation. A borrower incurs a line of credit or credit card obligation when the borrower first has the right to borrow against the line of credit or credit card, whether or not the borrower actually borrows an amount at that time.
So this appears to indicate the amount in box 2 should be amount of the line of credit.
In TurboTax the 1098 information you enter only goes on a worksheet to figure to the deductible portion of mortgage interest paid. Neither that worksheet nor a form 1098 is sent as part of your tax return. To get the most correct calculations in TurboTax you should enter the amount of your line of credit (or at least the amount you actually paid interest on).
As long as your average mortgage balances don't exceed one of the mortgage limitations ($750K for loans taken after Dec 2017 for example) then entering $1 will allow the calculations.
But if the total average balance of your loan(s) actually exceeded a limitation, then your deduction may need to be limited but may not be as you only entered $1 for the balance.
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