Ensure attestations are created with empty signature (#960)

* Ensure attestations are created with empty sig

* Update docs

beacon_node/beacon_chain/src/beacon_chain.rs

@@ -756,7 +756,7 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
                     root: target_root,
                 },
             },
-            signature: AggregateSignature::new(),
+            signature: AggregateSignature::empty_signature(),
         })
     }
 

beacon_node/beacon_chain/tests/attestation_production.rs

@@ -106,7 +106,7 @@ fn produces_attestations() {
             );
             assert_eq!(
                 attestation.signature,
-                AggregateSignature::new(),
+                AggregateSignature::empty_signature(),
                 "bad signature"
             );
             assert_eq!(data.index, index, "bad index");

book/src/http_validator.md

@@ -263,7 +263,7 @@ Returns a `Attestation` object with a default signature. The `signature` field s
             "root": "0xaefd23b384994dc0c1a6b77836bdb2f24f209ebfe6c4819324d9685f4a43b4e1"
         }
     },
-    "signature": "0xc00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000"
+    "signature": "0x000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000"
 }
 ```
 

eth2/utils/bls/src/aggregate_signature.rs

@@ -32,9 +32,12 @@ impl AggregateSignature {
 
     /// Add (aggregate) a signature to the `AggregateSignature`.
     pub fn add(&mut self, signature: &Signature) {
-        if !self.is_empty {
-            self.aggregate_signature.add(signature.as_raw())
+        if self.is_empty {
+            self.aggregate_signature = RawAggregateSignature::new();
+            self.is_empty = false;
         }
+
+        self.aggregate_signature.add(signature.as_raw())
     }
 
     /// Add (aggregate) another `AggregateSignature`.