BUG: Timestamp.isoformat works incorrect on ns precision

[i1oveMyse1f]

Pandas version checks

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• 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

import pandas as pd

ts = pd.Timestamp("2023-01-01 00:00:00")
ts_1ns = ts + pd.Timedelta(nanoseconds=1)

print(ts.isoformat(timespec='nanoseconds'))
print(ts_1ns.isoformat(timespec='nanoseconds'))

Issue Description

If the requested precision is in nanoseconds and the timestamp has 0 microseconds, then the format is invalid and looks like the following:

2023-01-01T00:00:00.000000.000000000
2023-01-01T00:00:00.000000.000000001

Expected Behavior

2023-01-01T00:00:00.000000000
2023-01-01T00:00:00.000000001

Installed Versions

/opt/homebrew/Cellar/python@3.11/3.11.2_1/Frameworks/Python.framework/Versions/3.11/lib/python3.11/site-packages/_distutils_hack/__init__.py:33: UserWarning: Setuptools is replacing distutils.
  warnings.warn("Setuptools is replacing distutils.")

INSTALLED VERSIONS
------------------
commit           : 37ea63d540fd27274cad6585082c91b1283f963d
python           : 3.11.2.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 22.4.0
Version          : Darwin Kernel Version 22.4.0: Mon Mar  6 20:59:28 PST 2023; root:xnu-8796.101.5~3/RELEASE_ARM64_T6000
machine          : arm64
processor        : arm
byteorder        : little
LC_ALL           : None
LANG             : ru_RU.UTF-8
LOCALE           : ru_RU.UTF-8

pandas           : 2.0.1
numpy            : 1.24.2
pytz             : 2023.3
dateutil         : 2.8.2
setuptools       : 65.6.3
pip              : 23.1
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 3.1.2
IPython          : 8.12.0
pandas_datareader: None
bs4              : 4.12.2
bottleneck       : None
brotli           : None
fastparquet      : None
fsspec           : None
gcsfs            : None
matplotlib       : 3.7.1
numba            : None
numexpr          : None
odfpy            : None
openpyxl         : 3.1.2
pandas_gbq       : None
pyarrow          : None
pyreadstat       : None
pyxlsb           : None
s3fs             : None
scipy            : 1.10.1
snappy           : None
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
zstandard        : None
tzdata           : 2023.3
qtpy             : None
pyqt5            : None