Count/generation algorithm uses empty cells. Should probably be an option to ignore.

The solution generation algorithm (and thus also the solution counter algorithm) uses cells even if they are empty. While this is sometimes preferable, it would be useful to easily choose to ignore empty cells.

Start by locating the solution generation and solution counter algorithms in the JavaScript project, then trace how cells are currently considered when empty. Determine where an option can apply to both algorithms and verify that generation and counting can either include or ignore empty cells.

Written by the indexing model from the issue text.