If you format the cell (showing the hyphen), then use:
Format|Cells|number tab
you'll see that the custom format is:
_($* #,##0.00_);_($* (#,##0.00);_($* "-"??_);_(@_)
You could change it to:
_($* #,##0.00_);_($* (#,##0.00);_($* 0.00_);_(@_)
If you want to fix all the cells formatted with the Currency style, you could
use:
Format|Style|Select Currency
Click Modify and change the numberformat to what you like.
Be aware that styles live in workbooks. You'll have to do the same thing for
each workbook that needs it.