Pandas BUG: Sql select from database type cast issue

Pandas version checks

• [x] I have checked that this issue has not already been reported.

• [x] I have confirmed this bug exists on the latest version of pandas.

• [ ] I have confirmed this bug exists on the main branch of pandas.

Reproducible Example

from pathlib import Path
import tempfile
import shutil
import datetime

import pandas as pd

from sqlalchemy import (
    create_engine, MetaData, Table, Column, Integer,
    Float, DateTime, select, insert
)


class DB:

    def __init__(self, db_file_path: str | Path):
        self.engine = create_engine(f"sqlite:///{db_file_path}", echo=False)
        self.metadata = MetaData()
        self._table = Table(
            "main_table", self.metadata,

            Column('time', DateTime, unique=True),
            Column('a', Float, nullable=False),
            Column('b', Float, nullable=False),
            Column('c', Integer, nullable=True),
            Column('d', Float, nullable=True),

            keep_existing=True
        )

        self.metadata.create_all(self.engine)

    def populate(self):
        data = [
            {"time": datetime.datetime.fromisoformat('2025-07-26T04:11:00Z'), "a": 1.1, "b": 1.2, "c": None, "d": None},
            {"time": datetime.datetime.fromisoformat('2025-07-26T05:22:00Z'), "a": 2.1, "b": 2.2, "c": None, "d": None},
            {"time": datetime.datetime.fromisoformat('2025-07-26T06:33:00Z'), "a": 3.1, "b": 3.2, "c": None, "d": None},
            {"time": datetime.datetime.fromisoformat('2025-07-26T07:44:00Z'), "a": 4.1, "b": 4.2, "c": None, "d": None},
            {"time": datetime.datetime.fromisoformat('2025-07-26T08:55:00Z'), "a": 5.1, "b": 5.2, "c": None, "d": None},
        ]
        with self.engine.connect() as conn:
            conn.execute(insert(self._table), data)
            conn.commit()

    def read_records(self):
        _select = select(self._table)
        with self.engine.connect() as conn:
            df = pd.read_sql(_select, conn)
        return df

    def __del__(self):
        self.engine.dispose()


if __name__ == '__main__':
    temp_folder = Path(tempfile.mkdtemp(prefix="pandas_bug_"))
    db_file_path = temp_folder / "collected_data_binance.db"
    print(db_file_path)
    if db_file_path.exists():
        db_file_path.unlink()

    db = DB(db_file_path=db_file_path)
    db.populate()
    df = db.read_records()
    print(df.info())

    del(db)
    shutil.rmtree(temp_folder)

Issue Description

Db types:

Db data:

After reading data using pd.read_sql(_select, conn) we see:

Expected Behavior

The columns with NULL should inherit type from column type of DB instead of "object"

Installed Versions

INSTALLED VERSIONS ------------------ commit : 2cc37625532045f4ac55b27176454bbbc9baf213 python : 3.13.3 python-bits : 64 OS : Windows OS-release : 11 Version : 10.0.22631 machine : AMD64 processor : Intel64 Family 6 Model 183 Stepping 1, GenuineIntel byteorder : little LC_ALL : None LANG : None LOCALE : English_United States.1252 pandas : 2.3.0 numpy : 1.26.4 pytz : 2025.2 dateutil : 2.9.0.post0 pip : 25.1.1 Cython : None sphinx : None IPython : None adbc-driver-postgresql: None adbc-driver-sqlite : None bs4 : None blosc : None bottleneck : None dataframe-api-compat : None fastparquet : None fsspec : 2025.5.1 html5lib : None hypothesis : None gcsfs : None jinja2 : 3.1.6 lxml.etree : None matplotlib : None numba : None numexpr : None odfpy : None openpyxl : None pandas_gbq : None psycopg2 : None pymysql : None pyarrow : 20.0.0 pyreadstat : None pytest : 8.4.1 python-calamine : None pyxlsb : None s3fs : None scipy : None sqlalchemy : 2.0.41 tables : None tabulate : 0.9.0 xarray : None xlrd : None xlsxwriter : None zstandard : None tzdata : 2025.2 qtpy : None pyqt5 : None