Skip combined device creation for empty spec list in DigitizerWorkflow

  With --combine-devices, specCombiner() is called unconditionally for
  digitizerSpecs and for writerSpecs, but when a collection is empty the result
  is a combined device with no sub-tasks: DPL sees a device with no inputs,
  injects a Lifetime::Enumeration input (WorkflowHelpers.cxx) and can elect
  it as the CCDB/DISTSUBTIMEFRAME source.

  No digits are produced, and since the enumeration is unbounded there is no
  EOS either, meaning that the workflow hangs forever instead of failing.

  This is reached with --onlyDet FT0,CTP, where the FT0 digitizer and the CTP
  digitizer are pushed to specs directly and only the FT0 digit writer goes
  to writerSpecs, leaving digitizerSpecs empty. In anchored MC that is the
  detector group of any run taken without both FV0 and EMCAL, such as run
  537401, where the problem was found.

Author: jackal1-66

Steer/DigitizerWorkflow/src/SimpleDigitizerWorkflow.cxx

@@ -850,8 +850,15 @@ WorkflowSpec defineDataProcessing(ConfigContext const& configcontext)
     }
   } else {
     std::vector<DataProcessorSpec> remaining;
-    specs.push_back(specCombiner("Digitizations", digitizerSpecs, remaining));
-    specs.push_back(specCombiner("Writers", writerSpecs, remaining));
+    // Skip combined device creation for an empty collection as specCombiner would
+    // return a device with no sub-tasks. This problem was discovered with run 537401 where both FT0,CTP detectors
+    // bypass digitizerSpecs and FV0 and EMC are not included in the run.
+    if (!digitizerSpecs.empty()) {
+      specs.push_back(specCombiner("Digitizations", digitizerSpecs, remaining));
+    }
+    if (!writerSpecs.empty()) {
+      specs.push_back(specCombiner("Writers", writerSpecs, remaining));
+    }
     for (auto& s : remaining) {
       specs.push_back(s);
     }