Write one empty TPC digit entry when a timeframe has no collision

This fixes the TPC digit writer producing a file without a tree when a
timeframe holds no collision.

- The custom close callback only called TFile::Close inside "if (entries >
  0)", and never called TFile::Write, so with nothing to write the tree
  never reached the file.
- The result was a 942 byte file with no o2sim tree, and every reader of it
  failed on a missing branch rather than on an empty tree.
- Each branch is now filled once with the empty default object it is bound
  to, so the file is an ordinary timeframe that happens to contain no digit
  and the readers downstream stay on their normal path.
- The tree is written explicitly, the way RootTreeWriter's own close does.

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

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

Author: sawenzel

Detectors/TPC/simworkflow/src/TPCDigitRootWriterSpec.cxx

@@ -69,12 +69,24 @@ DataProcessorSpec getTPCDigitRootWriterSpec(std::vector<int> const& laneConfigur
         LOG(warning) << "INCONSISTENT NUMBER OF ENTRIES IN BRANCH " << br->GetName() << ": " << entries << " vs " << brentries;
       }
     }
-    if (entries > 0) {
-      LOG(info) << "Setting entries to " << entries;
-      outputtree->SetEntries(entries);
-      // outputtree->Write("", TObject::kOverwrite);
-      outputfile->Close();
+    if (entries <= 0) {
+      // A timeframe holds no collision at all whenever the interaction rate is low enough, and
+      // then no branch is filled. Write one empty entry in every branch instead of nothing, so
+      // that the file is an ordinary timeframe that happens to contain no digit and every reader
+      // downstream stays on its normal path. Each branch is bound to a default constructed object
+      // of its own type by RootTreeWriter, so Fill() writes exactly that.
+      LOG(info) << "No branch was filled, writing one empty entry per branch";
+      for (TObject* entry : *brlist) {
+        static_cast<TBranch*>(entry)->Fill();
+      }
+      entries = 1;
     }
+    LOG(info) << "Setting entries to " << entries;
+    outputtree->SetEntries(entries);
+    // write the tree explicitly, the way RootTreeWriter's own close does. Closing the file alone
+    // leaves an empty tree without a key, so the file comes out with no tree in it at all.
+    outputfile->Write();
+    outputfile->Close();
   };
 
   // branch definitions for RootTreeWriter spec