Source add_spectra in case of 0 valid pixels from obj mask

in source.py:extract_spectra(), at line 1811 i had to add the test in case ksel has 0 valid pixels

        ksel = (object_mask != 0)

        if ksel.sum() == 0:
            self._logger.error('object mask has 0 valid pixels')
        else:
            for tag in nb_tags:
                self._logger.info("Extract spectra with tag %s" % (tag))
                if self.images[tag].wcs.isEqual(wcsref):
                    weight = self.images[tag].data.copy()
                    weight[ksel] -= np.min(weight[ksel])
                    weight = weight.filled(0)
                    self.spectra[tag + suffix] = compute_optimal_spectrum(
                        subcub, object_mask, weight)