BUG: `truncate` has wrong behavior when the Index has only one unique value
- [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.

- [x] (optional) I have confirmed this bug exists on the master branch of pandas.

---

#### Code Sample, a copy-pastable example

```python
# Returns an empty Series/DataFrame when the one you pass in has length > 1 and an index with only one unique value
x = pd.Series(0, index=pd.date_range('2021-06-29', '2021-06-29')).repeat(5)
x.truncate('2021-06-28', '2021-07-01')

# If you add another unique index value, you get the expected behavior
x = pd.Series(0, index=pd.date_range('2021-06-29', '2021-06-29')).repeat(5)
y = pd.Series(0, index=pd.date_range('2021-06-01', '2021-06-01'))
x = x.append(y)
x.truncate('2021-06-28', '2021-07-01')
```

#### Problem description
I would expect truncate to not do any filtering in the first case given that all index values are between `before` and `after`. The fix is trivial, so I'll submit a PR.

#### Expected Output
```
2021-06-29    0
2021-06-29    0
2021-06-29    0
2021-06-29    0
2021-06-29    0
```

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit           : 7c48ff4409c622c582c56a5702373f726de08e96
python           : 3.9.5.final.0
python-bits      : 64
OS               : Windows
OS-release       : 10
Version          : 10.0.19043
machine          : AMD64
processor        : Intel64 Family 6 Model 142 Stepping 10, GenuineIntel
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : English_United States.1252

pandas           : 1.2.5
numpy            : 1.20.2
pytz             : 2021.1
dateutil         : 2.8.1
pip              : 21.1.3
setuptools       : 52.0.0.post20210125
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 3.0.1
IPython          : 7.22.0
pandas_datareader: None
bs4              : None
bottleneck       : 1.3.2
fsspec           : None
fastparquet      : None
gcsfs            : None
matplotlib       : None
numexpr          : 2.7.3
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pyxlsb           : None
s3fs             : None
scipy            : None
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
numba            : None

</details>
