bls: debugging utils and fake sig fix

There was a discrepancy between the is_empty fields of fake signatures during testing, so I've added
a small hack to set the is_empty field of a fake signature based on the byte content.
Alternatively, we could just make it so that any fake signature is defined to be equal to any other.

eth2/utils/bls/src/fake_public_key.rs

@@ -1,6 +1,5 @@
 use super::{SecretKey, BLS_PUBLIC_KEY_BYTE_SIZE};
 use cached_tree_hash::cached_tree_hash_ssz_encoding_as_vector;
-use milagro_bls::PublicKey as RawPublicKey;
 use serde::de::{Deserialize, Deserializer};
 use serde::ser::{Serialize, Serializer};
 use serde_hex::{encode as hex_encode, HexVisitor};