fix: Let Root writers accept empty Trajectories object (#1864)

The Root Writers crash when they encounter an empty Trajectories object (e.g., from a failed fit) which should be fixed by this PR.

One thing that is not optimal is that we seem not to test a case with failing tracks, but I'm not sure how to setup such a test in the best way...

Examples/Io/Root/src/RootTrajectoryStatesWriter.cpp

@@ -265,10 +265,17 @@ ActsExamples::ProcessCode ActsExamples::RootTrajectoryStatesWriter::writeT(
     // The trajectory index
     m_multiTrajNr = itraj;
 
-    // The trajectory entry indices and the multiTrajectory
-    const auto& mj = traj.multiTrajectory();
+    // The trajectory entry indices
     const auto& trackTips = traj.tips();
 
+    // Dont write empty MultiTrajectory
+    if (trackTips.empty()) {
+      continue;
+    }
+
+    // The MultiTrajectory
+    const auto& mj = traj.multiTrajectory();
+
     // Loop over the entry indices for the subtrajectories
     for (unsigned int isubtraj = 0; isubtraj < trackTips.size(); ++isubtraj) {
       // The subtrajectory index

Examples/Io/Root/src/RootTrajectorySummaryWriter.cpp

@@ -168,15 +168,17 @@ ActsExamples::ProcessCode ActsExamples::RootTrajectorySummaryWriter::writeT(
   for (size_t itraj = 0; itraj < trajectories.size(); ++itraj) {
     const auto& traj = trajectories[itraj];
 
-    // The trajectory entry indices and the multiTrajectory
-    const auto& mj = traj.multiTrajectory();
+    // The trajectory entry indices
     const auto& trackTips = traj.tips();
 
     // Dont write empty MultiTrajectory
     if (trackTips.empty()) {
       continue;
     }
 
+    // Get the MultiTrajectory
+    const auto& mj = traj.multiTrajectory();
+
     // The trajectory index
     m_multiTrajNr.push_back(itraj);