IF formula needed to only show answer when number entered into another cell.

Please see attached WS

I currently have a formula in N6:N1005 that calculates a sales commission based on multiple percentage levels of the commission offered.
Basically, the commission is calculated as a percentage on the 1st $100,000 of the sales price (L6:L1005) and then another percentage on the balance (M6:M1005).
In my spreadsheet the sales price is F6:F1005.
The problem is, when I do not enter a sales price into F6:F1005, a negative number appears in N6:N1005 which I do not want (see N7).
I simply do not want the formula to calculate in N6:N1005 until an amount is entered into F6:F1005.
Please help, thank you.