Make a Formula Output a Blank Cell

Hi Everyone

Hope someone can help me with a problem I have.

I have 2 formulas

 =E2+(11*7)

and

=E2+(24*7)

in the cells of 2 columns, Columns G and H, that provides a date for the future dependant on dates input from the cells within a different Column, that being Column E

The problem I have is that when I autofill the formula down the cells of Columns G and H dates appear instead of empty cells.

How can I modify the formula so the cells in Columns G and H remain empty until a date is input in Column E ?

I have attached a stripped down version of the worksheet.

Thank You in advance

Ortz