GammaLib

If CTA events are simulated for an energy range where the effective area is zero the GModelSpectralNodes::append() method throws an exception.

The reason is that the code in GCTAModelAeffBackground::mc

        GModelSpectralNodes spectral;
        for (int i = 0; i < spectral_ebounds.size(); ++i) {
            GEnergy energy   = spectral_ebounds.elogmean(i);
            double intensity = aeff_integral(obs, energy.log10TeV());
            double  norm     = m_spectral->eval(energy, events->tstart());
            spectral.append(energy, norm * intensity);
        }

does not check whether the product norm * intensity is positive, which leads to the exception. A test should be added that appends only positive nodes.

The side effect is that the GModelSpectralNodes object may be empty, which later poses a problem in GModelSpectralNodes::flux that returns an exception if there are no nodes. Instead, the method should simply return zero.