Write a blank cell to the specified row and column (zero indexed). A blank cell is used to specify formatting without adding a string or a number. A blank cell without a format serves no purpose. Therefore, we don't write a BLANK record unless a format is specified. This is mainly an optimisation for the write_row() and write_col() methods. Returns 0 : normal termination (including no format) -1 : insufficient number of arguments -2 : row or column out of range