Can you set CHAR to NULL or an empty string?

I haven't used CHAR before, since it seems like I've used VARCHAR too often; I am trying to use CHAR when this comes up lately, and from what I understand you are using when the data is the same length in a specific column (otherwise they are filled with spaces).

Since all data should be the same length, I was wondering if the CHAR field could be NULL or an empty string? Such cases when this particular field does not matter while others do.