Fixed: when assume_consistent, Solver::Assign() must not test [].

The semantics within G modalities is that the set of literals in the
grounded clause plus the transitive closure of literals occurring in
clauses in UP^-(s) is consistent.

In the extreme case where UP^-(s) contains [], this means only the
literals in the clause are checked. This particular extreme case is
somewhat counterintuitive because the trivial answer is 'no', but it
simplifies the definition.