Bug fix in the LEFT JOIN strength reduction optimization of

check-in [dd568c27b1d76563].  The sqlite3ExprImpliesNotNull() routine was
mistakenly assuming that a CASE expression must always be NULL if contained
any reference to a variable that was NULL.

FossilOrigin-Name: cf171abe954a5f25262161dd69f2e8cecdbf9446c3f6b298201507dbc743567e