Iterating through a datagridview

Sir/Ma'am
I was hoping one of you could assist me with iterating through a datagridview to find null values.  Basically, I have a datagridview consisting of 14 fixed columns.  If the user adds a new row and doesn't populate all 14 cells for that specific row, I would like to input the value of "00000" into the unpopulated cells.  The code below will simply write the data in the cells to a txt file, which is fine until I need to import said data into my datagridview.