Add empty() to grapht as it implements size(), bug-fix

Follow-up to fix build failure introduced in diffblue#1032/9d991b4

Author: tautschnig

src/solvers/sat/satcheck_minisat2.cpp

@@ -155,7 +155,7 @@ propt::resultt satcheck_minisat2_baset<T>::prop_solve()
         {
           messaget::status() <<
             "SAT checker: instance is SATISFIABLE" << eom;
-          assert(!solver->model.empty());
+          CHECK_RETURN(solver->model.size()>0);
           status=statust::SAT;
           return resultt::P_SATISFIABLE;
         }

src/util/graph.h

@@ -150,6 +150,11 @@ class grapht
     return nodes.size();
   }
 
+  bool empty() const
+  {
+    return nodes.empty();
+  }
+
   const edgest &in(node_indext n) const
   {
     return nodes[n].in;