Support for conjunction over empty set

src/storm-dft/modelchecker/dft/DFTASFChecker.cpp

@@ -107,11 +107,15 @@ namespace storm {
 
             std::string toSmtlib2(std::vector<std::string> const& varNames) const override {
                 std::stringstream sstr;
+                if (constraints.empty()) {
+                    sstr << "true";
+                } else {
                     sstr << "(and";
                     for(auto const& c : constraints) {
                         sstr << " " << c->toSmtlib2(varNames);
                     }
                     sstr << ")";
+                }
                 return sstr.str();
             }