Let the MCH digitiser accept a timeframe without collisions

This fixes a segmentation fault in MCHDPLDigitizerTask when the collision
context of a timeframe is empty.

- The noise-only signal range was taken from eventRecords.front() and
  eventRecords.back(), which is undefined behaviour on an empty vector.
- A timeframe holds no collision whenever the interaction rate is low.
- The range now comes from HBFUtils in that case, so the noise covers the
  timeframe that is actually being digitised.

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

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

Author: sawenzel

Steer/DigitizerWorkflow/src/MCHDigitizerSpec.cxx

@@ -15,6 +15,7 @@
 #include "DataFormatsMCH/ROFRecord.h"
 #include "DataFormatsParameters/GRPObject.h"
 #include "DetectorsBase/BaseDPLDigitizer.h"
+#include "DetectorsRaw/HBFUtils.h"
 #include "Framework/ConfigParamRegistry.h"
 #include "Framework/ControlService.h"
 #include "Framework/DataProcessorSpec.h"
@@ -102,9 +103,20 @@ class MCHDPLDigitizerTask : public o2::base::BaseDPLDigitizer
       }
     }
 
-    // generate noise-only signals between first and last collisions ± 100 BC (= 25 ADC samples)
-    auto firstIR = InteractionRecord::long2IR(std::max(int64_t(0), eventRecords.front().toLong() - timeOffset - 100));
-    auto lastIR = InteractionRecord::long2IR(std::max(int64_t(0), eventRecords.back().toLong() - timeOffset + 100));
+    // generate noise-only signals between first and last collisions ± 100 BC (= 25 ADC samples).
+    // A timeframe can hold no collision at all when the interaction rate is low; take the range
+    // from the timeframe itself in that case, since there are no collisions to take it from.
+    int64_t firstLong, lastLong;
+    if (eventRecords.empty()) {
+      const auto& hbf = o2::raw::HBFUtils::Instance();
+      firstLong = InteractionRecord(0, hbf.orbitFirstSampled).toLong();
+      lastLong = InteractionRecord(0, hbf.orbitFirstSampled + hbf.nHBFPerTF).toLong();
+    } else {
+      firstLong = eventRecords.front().toLong();
+      lastLong = eventRecords.back().toLong();
+    }
+    auto firstIR = InteractionRecord::long2IR(std::max(int64_t(0), firstLong - timeOffset - 100));
+    auto lastIR = InteractionRecord::long2IR(std::max(int64_t(0), lastLong - timeOffset + 100));
     mDigitizer->addNoise(firstIR, lastIR);
 
     // digitize