fix: Fix RootSimHitReader for empty events (#2478)

Looking at the writer this can happen for non recorded hits so the
reader should also support that.

I discovered this while simulating single particles with Geant4 in our
Examples framework

---------

Co-authored-by: kodiakhq[bot] <49736102+kodiakhq[bot]@users.noreply.github.com>

Examples/Io/Root/src/RootSimHitReader.cpp

@@ -112,9 +112,20 @@ ActsExamples::ProcessCode ActsExamples::RootSimHitReader::read(
       m_eventMap.begin(), m_eventMap.end(),
       [&](const auto& a) { return std::get<0>(a) == context.eventNumber; });
 
-  if (m_inputChain == nullptr || it == m_eventMap.end()) {
-    ACTS_ERROR("Cannot read hits of event " << context.eventNumber);
-    return ActsExamples::ProcessCode::ABORT;
+  if (it == m_eventMap.end()) {
+    // explicitly warn if it happens for the first or last event as that might
+    // indicate a human error
+    if ((context.eventNumber == availableEvents().first) &&
+        (context.eventNumber == availableEvents().second - 1)) {
+      ACTS_WARNING("Reading empty event: " << context.eventNumber);
+    } else {
+      ACTS_DEBUG("Reading empty event: " << context.eventNumber);
+    }
+
+    m_outputSimHits(context, {});
+
+    // Return success flag
+    return ActsExamples::ProcessCode::SUCCESS;
   }
 
   // lock the mutex