A dash or hyphen is a symbol commonly used in tables to represent absence of information. Depending on context, it is an abbreviation for "missing data", "not applicable", or even "exactly zero".
There is no single correct character (glyph) to use, because there is no prevalent usage or any kind of standard. The symbol used might be a hyphen, an en-dash, or an em-dash.
It is true that a two-em dash indicates where some letters have been left out of a word, and a three-em dash indicates where a word has been entirely left out. But you would be hard pressed to find an example of this in a table.
Therefore, the choice of symbol is left up to you. Simply choose the one which makes your table easiest to read.