Unity Sprite Editor : Include blank/empty cells when slicing?

Actually you can. You'll just need to fill your empty cells with a solid color before slicing them for the first time. After that, just clear your main file and it will update as you go.

Just figured out a great way to do this. Before you slice your sprite, go to the sprite settings and for "Alpha Source" select None. Now, when you go to slice your sprite, Unity will think it's all one color and you can do a grid slice that will include empty cells. After you've sliced it, go back to the alpha source and select "Input Texture Alpha" and you're done!

No, you cannot automatically slice empty cells, as far as I know.

However, why would you want to? That would literally increase your VRAM usage with empty, useless data. If you want empty sprites, why not just define one, and use that where you need something to be invisible, or better yet, disable the component rendering it?

When in the Sprite Editor you can manually create / drag a frame with your mouse. I'm sure there is a better way but I haven't found it yet.