update/upsert sets a String field with "0"

Hi,

Up on record update/upsert (if update) new value is discarded and column value in db gets "0".

 CREATE TABLE `t1` (
  `pk` int(11) NOT NULL,
  `vl` varchar(200) NOT NULL,
  PRIMARY KEY (`pk`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1

:set +m
:set -XQuasiQuotes
:set -XOverloadedStrings
:set -XTemplateHaskell
:set -XTypeFamilies
:set -XGADTs
:set -XMultiParamTypeClasses
:set -XGeneralizedNewtypeDeriving

import Database.Persist
import Control.Monad.Logger
import Data.Pool
import Database.Persist.TH
import Database.Persist.MySQL

cinf = mkMySQLConnectInfo "localhost" "root" "root" "db1"
p2 <- runNoLoggingT (createMySQLPool cinf 2  :: NoLoggingT IO (Pool SqlBackend))

$(mkPersist sqlSettings  ([persistLowerCase|
MyT8 sql=t1 
  uid Int sql=pk
  value String sql=vl
  Primary uid
  UniqueUid uid
  deriving Show Read Eq Ord  |]));

# runSqlPool (upsert  (MyT8 11 "xxx") [MyT8Value +=. "yyy"]) p2
Entity {entityKey = MyT8Key {unMyT8Key = 11}, entityVal = MyT8 {myT8Uid = 11, myT8Value = "xxx"}}
# runSqlPool (upsert  (MyT8 11 "xxx") [MyT8Value +=. "yyy"]) p2
Entity {entityKey = MyT8Key {unMyT8Key = 11}, entityVal = MyT8 {myT8Uid = 11, myT8Value = "0"}}

Checking db

mysql> select * from t1 where pk = 11;
+----+----+
| pk | vl |
+----+----+
| 11 | 0  |
+----+----+
1 row in set (0.00 sec)

GHCI 8.6.5 windows 10 and mysql 5.6.39 on docker

mysql-haskell-0.8.4.2
persistent-2.9.2

Start by running the supplied GHCi reproduction against the shown MySQL schema, comparing the initial insert with the repeated upsert. Trace the persistent upsert path for the String field and add coverage for this case; done means the repeated operation no longer stores "0" unexpectedly in t1.vl.