Segmentation fault for length zero frame list

[PicoCentauri]

Expected behavior

When providing an empty frames list to a SOAP representation the method get_feature method should raise a ValueError.

Actual behavior

The kernel quits with a segmentation fault.

Code to reproduce the behavior

from rascal.representations import SphericalInvariants as SOAP

hypers = dict(soap_type='PowerSpectrum',
              interaction_cutoff=3.5,
              max_radial=6,
              max_angular=6,
              gaussian_sigma_type='Constant',
              gaussian_sigma_constant=0.4,
              cutoff_smooth_width=0.5)

frames = []

soap = SOAP(**hypers)
soap_rep = soap.transform(frames)
soap_rep.get_features(soap)