slight improvement to GaussianMixtureFactor print

gtsam/hybrid/GaussianMixtureFactor.cpp

@@ -57,11 +57,12 @@ void GaussianMixtureFactor::print(const std::string &s,
       [&](const GaussianFactor::shared_ptr &gf) -> std::string {
         RedirectCout rd;
         std::cout << ":\n";
-        if (gf)
+        if (gf && !gf->empty()) {
           gf->print("", formatter);
-        else
-          return {"nullptr"};
           return rd.str();
+        } else {
+          return "nullptr";
+        }
       });
   std::cout << "}" << std::endl;
 }