Use nil instead of protobuf.Empty in genesis BaseAccount (#7463)

* Use nil instead of protobuf.Empty in genesis

* Fix tests

x/auth/legacy/v040/migrate.go

@@ -1,8 +1,6 @@
 package v040
 
 import (
-	pt "github.com/gogo/protobuf/types"
-
 	codectypes "github.com/cosmos/cosmos-sdk/codec/types"
 	sdkerrors "github.com/cosmos/cosmos-sdk/types/errors"
 	v039auth "github.com/cosmos/cosmos-sdk/x/auth/legacy/v039"
@@ -13,21 +11,16 @@ import (
 
 // convertBaseAccount converts a 0.39 BaseAccount to a 0.40 BaseAccount.
 func convertBaseAccount(old *v039auth.BaseAccount) *v040auth.BaseAccount {
-	// In `x/auth/legacy/v038/migrate.go`, when creating a BaseAccount, we
-	// explicitly set the PublicKey field to nil. This propagates until 0.40,
-	// and we don't know the PubKey for those accounts, so we just put an empty
-	// string inside the Any.
 	var any *codectypes.Any
-	var err error
+	// If the old genesis had a pubkey, we pack it inside an Any. Or else, we
+	// just leave it nil.
 	if old.PubKey != nil {
+		var err error
 		any, err = tx.PubKeyToAny(old.PubKey)
-	} else {
-		s := pt.Empty{}
-		any, err = codectypes.NewAnyWithValue(&s)
-	}
 		if err != nil {
 			panic(err)
 		}
+	}
 
 	return &v040auth.BaseAccount{
 		Address:       old.Address.String(),

x/auth/legacy/v040/migrate_test.go

@@ -228,10 +228,7 @@ func TestMigrate(t *testing.T) {
       "@type": "/cosmos.auth.v1beta1.BaseAccount",
       "account_number": "1",
       "address": "cosmos16ydaqh0fcnh4qt7a3jme4mmztm2qel5axcpw00",
-      "pub_key": {
-        "@type": "/google.protobuf.Empty",
-        "value": {}
-      },
+      "pub_key": null,
       "sequence": "0"
     }
   ],

x/auth/types/codec.go

@@ -19,7 +19,7 @@ func RegisterLegacyAminoCodec(cdc *codec.LegacyAmino) {
 	legacytx.RegisterLegacyAminoCodec(cdc)
 }
 
-// RegisterInterface associates protoName with AccountI interface
+// RegisterInterfaces associates protoName with AccountI interface
 // and creates a registry of it's concrete implementations
 func RegisterInterfaces(registry types.InterfaceRegistry) {
 	registry.RegisterInterface(