Can't sort DataFrame when level=[] and sort_remaining=False
#### Code Sample

```python
import pandas as pd

# Create a DataFrame with a MultiIndex.
df = pd.DataFrame({
    "a": [0, 2, 1],
    "b": [3, 0, 2],
    "c": [0, 0, 1]
}).set_index(["a", "b"])

# This works.
df.sort_index(level=[], sort_remaining=True)

# This throws an IndexError: list index out of range. 
df.sort_index(level=[], sort_remaining=False)
```

#### Problem description

Pandas is unable to sort the index of a DataFrame when level=[] and sort_remaining=False. It works when sort_remaining=True, though. This is inconsistent behavior; it doesn't make sense for one to work but not the other.

The above code worked as intended in 0.22, but started throwing an error in 0.23. I believe that the commit that broke it was [this one](https://github.com/pandas-dev/pandas/commit/bc37ea2e05019a89adaa48159b220483598d1898#diff-1e79abbbdd150d4771b91ea60a4e1cc7). By changing 
```
if level:
```
to 
```
if level is not None:
```
the conditional is now entered when `level=[]`, whereas it wasn't previously (since an empty list is treated as `False`).


#### Expected Output

The offending line of code should be equivalent to `df`.

(You might wonder how the above situation would even arise; no one would ever deliberately write the above line of code, since it is a no-op. But it can happen when the `level=` argument is being programmatically generated.)

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.8.final.0
python-bits: 64
OS: Darwin
OS-release: 16.5.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.24.1
pytest: 3.3.2
pip: 18.1
setuptools: 38.4.0
Cython: 0.27.3
numpy: 1.15.4
scipy: 1.2.1
pyarrow: None
xarray: None
IPython: 6.2.1
sphinx: 1.6.6
patsy: 0.5.0
dateutil: 2.6.1
pytz: 2017.3
blosc: None
bottleneck: 1.2.1
tables: 3.4.2
numexpr: 2.6.9
feather: None
matplotlib: 2.1.2
openpyxl: 2.4.10
xlrd: 1.1.0
xlwt: 1.2.0
xlsxwriter: 1.0.2
lxml.etree: 4.1.1
bs4: 4.6.0
html5lib: 1.0.1
sqlalchemy: 1.2.1
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
gcsfs: None

</details>
