gui: render empty string for numeric system missing value

gui: render empty string for numeric system missing value

Numeric cells which contain the system missing value are currently rendered
as a number of spaces, a dot and a number of spaces depending on the
current variable width and decimal places settings. When the cell is
edited, then this string is pretty confusing.
With the patch system missing values for numeric variables are rendered
as empty strings. Editing the cell is then straightforward and starts
with an empty cell.