After inserting data with precompiled SQL, when queried, certain fields have additional null characters ('\x00') appended at the end

**Bug Description**
Service Version: Code from the main branch after version 0.8.4, compiled and deployed on macOS 13.5.1.
Client Version: The current project branch python-sdk, accessing the database using the dbapi method.

After inserting data with precompiled SQL, when queried, certain fields have additional null characters ('\x00') appended at the end, as shown in the fourth column:
`
[(1001, '2022-05-01', 'province1', 'city1\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00', 1, 1590738991000),
(1002, '2022-05-02', 'province2', 'city2\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00', 2, 1590738992000),
(1003, '2022-05-03', 'province3', 'city3\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00', 3, 1590738993000),
(1004, '2022-05-04', 'province0', 'city4\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00', 4, 1590738994000)]
`

**Expected Behavior**
The field contents should not have additional null characters appended and should match the inserted content.

**Relation Case**

**Steps to Reproduce**

1. Use the python-sdk code from the main branch after version 0.8.4.
2. Run the test cases below to reproduce the corresponding error.

```
import sys
import os
from openmldb.dbapi import connect
import pytest

from .case_conf import OpenMLDB_ZK_CLUSTER, OpenMLDB_ZK_PATH

class TestOpenmldbDBAPI:
cursor_without_db = None
cursor = None
db_name = "dbapi_test"

@classmethod
def setup_class(cls):
# connect without db to create it
db = connect(zk=OpenMLDB_ZK_CLUSTER, zkPath=OpenMLDB_ZK_PATH)
cls.cursor_without_db = db.cursor()
cls.cursor_without_db.execute(
f"create database if not exists {cls.db_name};")
# then we can use the db to connect
db = connect(database=cls.db_name,
zk=OpenMLDB_ZK_CLUSTER,
zkPath=OpenMLDB_ZK_PATH)
cls.cursor = db.cursor()
cls.cursor.execute("create database if not exists {};".format(cls.db_name))
assert cls.db_name in cls.cursor.get_databases()

def recreate_table(self, table, schema):
if table in self.cursor.get_tables(self.db_name):
self.cursor.execute("drop table {}".format(table))
assert table not in self.cursor.get_tables(self.db_name)

self.cursor.execute("create table {}({}) OPTIONS(partitionnum=1);".format(table, schema))
assert table in self.cursor.get_tables(self.db_name)

def test_more_parameterized_query(self):
table = "test_param_query"
schema_str = "col1 bigint, col2 date, col3 string, col4 string, col5 int, col6 timestamp, " \
"index(key=col3, ts=col1), index(key=col3, ts=col6)"
self.recreate_table(table, schema_str)
test_rows = [(1000 + i, '2022-05-0' + str(i), 'province' + str(i % 4),
'city' + str(i), i, (1590738990 + i) * 1000)
for i in range(1, 5)]
self.cursor.executemany('insert into {} values (?, ?, ?, ?, ?, ?);'.format(table), test_rows)

result = self.cursor.execute("select * from {};".format(table)).fetchall()
result = sorted(list(result), key=lambda x: x[0])
assert result == test_rows

if __name__ == "__main__":
sys.exit(pytest.main(["-vv", os.path.abspath(__file__)]))
```

El issue incluye un caso de prueba de Python que falla y que inserta y consulta datos mediante el OpenMLDB Python SDK. Empieza ejecutando la prueba proporcionada para reproducir el relleno con caracteres nulos. Examina cómo el motor de base de datos de C++ gestiona los campos de tipo string en SQL precompilado, especialmente la asignación de memoria y la lógica de relleno. Comprueba si la capa dbapi del Python SDK presenta problemas de codificación/decodificación de strings. El objetivo es garantizar que los strings se devuelvan sin caracteres nulos adicionales.

Escrito por el modelo de indexación a partir del texto del issue.