Give the ITS and MFT digit file its usual shape when it is empty

This fixes the digit file of a timeframe without collisions carrying a label
branch of the wrong type.

- The MC label branch is declared as std::vector<char> and only becomes an
  IOMCTruthContainerView when a fill remaps it.
- With no collision nothing is filled, the branch keeps the raw type, and a
  reader binding IOMCTruthContainerView gets a class mismatch from
  SetBranchAddress rather than an empty tree.
- The close callback now writes one empty entry in every branch, remapping
  the label branch on the way, so the file has the same shape as an ordinary
  timeframe that happens to contain nothing.

https://its.cern.ch/jira/browse/O2-7132

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: sawenzel

Detectors/ITSMFT/common/workflow/src/DigitWriterSpec.cxx

@@ -23,6 +23,7 @@
 #include "DataFormatsITSMFT/ROFRecord.h"
 #include "SimulationDataFormat/ConstMCTruthContainer.h"
 #include "SimulationDataFormat/IOMCTruthContainerView.h"
+#include "Framework/Logger.h"
 #include "SimulationDataFormat/MCCompLabel.h"
 #include <vector>
 #include <string>
@@ -73,6 +74,31 @@ DataProcessorSpec getDigitWriterSpec(bool mctruth, bool doStag, bool dec, bool c
       }
       nent = n;
     }
+    if (nent == 0) {
+      // A timeframe holds no collision at all whenever the interaction rate is low enough, and no
+      // branch is then filled. Write one empty entry in every branch, so that the file has the same
+      // shape as an ordinary timeframe that happens to contain nothing. The label branch matters
+      // here: it is declared as std::vector<char> and only becomes an IOMCTruthContainerView when a
+      // fill remaps it, so without this a reader binding that type gets a class mismatch instead of
+      // an empty tree.
+      LOG(info) << "No branch was filled, writing one empty entry per branch";
+      std::vector<TBranch*> branches;
+      for (auto* o : *brArr) {
+        branches.push_back((TBranch*)o);
+      }
+      o2::dataformats::IOMCTruthContainerView emptyLabels;
+      auto* labelptr = &emptyLabels;
+      for (auto* br : branches) {
+        if (TString(br->GetName()).Contains("MCTruth")) {
+          auto* remapped = framework::RootTreeWriter::remapBranch(*br, &labelptr);
+          remapped->Fill();
+          remapped->ResetAddress();
+        } else {
+          br->Fill();
+        }
+      }
+      nent = 1;
+    }
     outputtree->SetEntries(nent);
     // do not use TTree::Write .. as this writes to default directory (not the associated file)
     // instead of outputtree->Write("", TObject::kOverwrite)