add warning for empty bins in ClusterEnsemble

[m-aguena]

Closes issue #618 , second interaction

[coveralls]

coverage: 100.0%. remained the same
when pulling fa84ad6 on issue/618/empty_radial_bin_again
into cfa7f11 on main.

[marina-ricci]

Just to copy here what we discussed : we want to return NaN instead of 0 when there are empty bins.

[m-aguena]

Just to copy here what we discussed : we want to return NaN instead of 0 when there are empty bins.

done!