2 choices
Cut a square, single sided bullnose piece to fit, and take a tile stone or grinder to it to round over the edge by 'hand', but that leaves an unglazed edge.
or
Cut a right and a left edge bullnose piece at a 45* and butt joint them across the diagonal middle of the corner. You could cut a right side 45* bullnose off the right end of 1 of your bullnose pieces and cut the left 45* piece of the left end of the same piece.
You'll need to be very careful when cutting the 45s, you dont want to fracture the pieces along the 45* seam. That'll make it stand out even more the diagonal seam will already...
Assuming that the missing piece needs to be perfectly square. It the countertop corner is slightly off after tiling, you may need to make 2 44* or 46* pieces, but make them the same....
Like this:
If you can cut them cleaning, you could leave a typical sized grout joints between the 2 45* pieces.
Good luck