Missing check in public key aggregation

[moCello]

Summary

From audit finding BLS-04:

For the same reasons as in BLS-03 (#7), the public key aggregation function
omits the identity check, though will do the subgroup check when
instantiated with from_bytes():

    pub fn aggregate(&mut self, pks: &[PublicKey]) {
        #[cfg(feature = "parallel")]
        let iter = pks.par_iter();

        #[cfg(not(feature = "parallel"))]
        let iter = pks.iter();

        let sum: G2Projective = iter
            .map(|pk| dusk_bls12_381::G2Projective::from(pk.pk_t()))
            .sum();
        (self.0).0 = ((self.0).0 + sum).into();
    }

Likewise, is_valid() may be used, or just is_identity().

Note that the signature aggregation does not require an identity check.