Resolve "TSHistogramsExporter doesn't export counters for merged regions"

Release notes

Details

The creation of projected histograms is turned off for combined regions: this is a safety measure since any kind of regions can be combined there (different x-axis or even different observables).

if(!region->isCombined()){
    projected               = region->createIntegral("_projected",normSet,projectedVars);
  }

so this means that the projection is NULL for combined reason.

I removed the if in order to get the counters for the combined regions.

else if(!f){
    throw std::runtime_error("NULL function passed to RooPdfEvaluator!");
  }

Closes #348 (closed)