In Excel, by Sorting the blank character should come first

There's no way to do that without a helper column that would have values in it. In the column next to all of this put the formula =A1="" this will return a false for non-blanks and a true for blanks. Then do custom sort, first by the true/false then ascending for the other value

It's a pain, but you can highlight your data, then press F5, select special, then select blanks. You can then color them or fill them with the single space others have suggested by hitting F2, then typing your space, and holding CTRL when you press enter.

I recommend color though, as you don't actually have to modify your data in a way that won't matter if you aren't otherwise using color to identify your data.