track empty clause during pop

If a theory solver creates the empty clause it gets dropped during pop.
By maintaining a variable m_empty_clause, the solver ensures that it retains the information that the search state is inconsistent.

src/smt/smt_internalizer.cpp

@@ -1351,6 +1351,7 @@ namespace smt {
                 m_justifications.push_back(j);
             TRACE("mk_clause", tout << "empty clause... setting conflict\n";);
             set_conflict(j == nullptr ? b_justification::mk_axiom() : b_justification(j));
+            m_empty_clause = true;
             SASSERT(inconsistent());
             return nullptr;
         case 1: