Pattern: fix solver's handling of empty rows.

Pattern: fix solver's handling of empty rows.

The algorithm for deducing how many squares in a row could be filled
in just from the initial clue set was focusing solely on _black_
squares, and forgot that if a row has a totally empty clue square then
everything in it can be filled in as white!

Now the solver can cope with puzzles such as 3x3:/1///1/ , where it
would previously have spuriously considered that it had no idea where
to start.