show in the JSON export whether a passphrase is empty or not

  ```json
  $ export-wallets --mainnet --keyfile state-wallet-mainnet/secret.key
  [
      {
          "encrypted_root_private_key": "18b952a968ccbac047f9c227dd05fced84ba85a1f83daf54f0a3c0de3e5664502678b1ba34a0b44ce22a2b54f39830570bdfeeed0a2f8586aa988fa5399987252b81f30b3c1e4c39e1edb16a4c7975eea106fc5d12d3b4fa516b9332f777defba949a42ef709a2d2f5c64057e99987c545b6e014f8cd9502bd5b800ddc6e10b7",
          "is_passphrase_empty": true,
          "name": null,
          "id": "f1aaaf1e1e6db1255ec2d59d50c5db892ff20330",
          "passphrase_hash": "31347c387c317c5743424875746242496c6a66734d764934314a30727a79663076657375724954796376766a793150554e377452673d3d7c54753434596d6e547957546c5759674a3164494f7974474a7842632b432f7862507657382b5135356a38303d"
      },
      {
          "encrypted_root_private_key": "6d9e6a5135d373fb62eadaf02bc8f5ecf35148806721c704c6dbfcf818ff958f1fc4a933542c423f5b8621cfe6404c4b208f2c96f6246082dffbc9fb1029a79ed33651377731e0e59e0233425a55257782c5adaa768da0567f43c1c6c0c18766ed0a547bb34eb472c120b170a8640279832ddf18002887f03c15dea59705422d",
          "is_passphrase_empty": false,
          "name": null,
          "id": "05377e480bd68a2da04f00c39ce5886e4746ed1f",
          "passphrase_hash": "31347c387c317c574342652b796362417576356c2b4258676a344a314c6343675375414c2f5653393661364e576a2b7550766655513d3d7c2f376738486c59723174734e394f6e4e753253302b6a65515a6b5437316b45414941366a515867386539493d"
      }
  ]
  ```

Author: KtorZ

wallet/export-wallets/Main.hs

@@ -24,6 +24,7 @@ import           Pos.Core.NetworkMagic (makeNetworkMagic)
 import           Pos.Crypto (EncryptedPass (..), EncryptedSecretKey (..))
 import           Pos.Crypto.Configuration (ProtocolMagic (..),
                      ProtocolMagicId (..), RequiresNetworkMagic (..))
+import           Pos.Crypto.Signing (checkPassMatches, emptyPassphrase)
 import           Pos.Infra.InjectFail (mkFInjects)
 import           Pos.Util.Trace (Trace (..))
 import           Pos.Util.UserSecret (readUserSecret)
@@ -97,11 +98,14 @@ extractWallet pw = do
 newtype Export a = Export a deriving (Show)
 
 instance ToJSON (Export (Maybe WalletName, EncryptedSecretKey)) where
-    toJSON (Export (name, EncryptedSecretKey{eskPayload, eskHash})) = Json.object
+    toJSON (Export (name, esk@EncryptedSecretKey{eskPayload, eskHash})) = Json.object
         [ "id" .= base16 (mkWalletId eskPayload)
         , "name" .= (getWalletName <$> name)
         , "encrypted_root_private_key" .= base16 (unXPrv eskPayload)
         , "passphrase_hash" .= base16 (getEncryptedPass eskHash)
+        , "is_passphrase_empty" .= case checkPassMatches emptyPassphrase esk of
+            Nothing -> False
+            Just{}  -> True
         ]
       where
         base16 = T.decodeUtf8 . convertToBase @ByteString @ByteString Base16