How would I combine these two formulas to give me an average of non empty
cells in a range and when copied down to not display a #DIV/0 error.

This formula takes care of the "not display a #DIV/0 error".
=AVERAGE(IF(ISERR(B15:H15),"",B15:H15))

And, this formula takes care of averaging the non-empty cells.
=AVERAGE(IF(B15:H15<0,B15:H15,FALSE))

Thank you