BUG: get_indexer_non_unique does not handle np.datetime64("NaT") and np.timedelta64("NaT")

[alexhlim]

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

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

• I have confirmed this bug exists on the master branch of pandas.

Reproducible Example

import numpy as np
from pandas import Index
import pandas._testing as tm


index = Index(
    np.array(
        [
            "2021-10-02",
            np.datetime64("NaT"),
            np.timedelta64("NaT"),
            np.datetime64("NaT"),
        ],
        dtype=object,
    )
)
# pass as index to prevent target from being casted to DatetimeIndex
indexer, missing = index.get_indexer_non_unique(
    Index([np.datetime64("NaT")], dtype=object)
)

expected_indexer = np.array([1, 3], dtype=np.intp)
tm.assert_numpy_array_equal(indexer, expected_indexer)

Issue Description

Index.get_indexer_non_unique() does not handle when multiple np.datetime64("NaT") or np.timedelta64("NaT") are present. Extension of #35498

Expected Behavior

See Reproducible Example

Installed Versions

INSTALLED VERSIONS

commit : 24ccae7
python : 3.8.10.final.0
python-bits : 64
OS : Darwin
OS-release : 20.6.0
Version : Darwin Kernel Version 20.6.0: Mon Aug 30 06:12:21 PDT 2021; root:xnu-7195.141.6~3/RELEASE_X86_64
machine : x86_64
processor : i386
byteorder : little
LC_ALL : None
LANG : en_US.UTF-8
LOCALE : en_US.UTF-8

pandas : 1.2.0.dev0+4209.g24ccae7031
numpy : 1.20.3
pytz : 2021.1
dateutil : 2.8.1
pip : 21.1.2
setuptools : 49.6.0.post20210108
Cython : 0.29.23
pytest : 6.2.4
hypothesis : 6.13.14
sphinx : 3.5.4
blosc : None
feather : None
xlsxwriter : 1.4.3
lxml.etree : 4.6.3
html5lib : 1.1
pymysql : None
psycopg2 : None
jinja2 : 2.11.3
IPython : 7.24.1
pandas_datareader: None
bs4 : 4.9.3
bottleneck : 1.3.2
fsspec : 2021.05.0
fastparquet : 0.6.3
gcsfs : 2021.05.0
matplotlib : 3.4.2
numexpr : 2.7.3
odfpy : None
openpyxl : 3.0.7
pandas_gbq : None
pyarrow : 4.0.0
pyxlsb : None
s3fs : 2021.05.0
scipy : 1.6.3
sqlalchemy : 1.4.17
tables : 3.6.1
tabulate : 0.8.9
xarray : 0.18.2
xlrd : 2.0.1
xlwt : 1.3.0
numba : 0.53.1

[alexhlim]

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