Reading a SAS file with 0 rows gives None
#### Code Sample, a copy-pastable example if possible

```python
In [2]: import pandas as pd

In [3]: pd.read_csv("zero_observations.csv")
Out[3]: 
Empty DataFrame
Columns: [a, b, c, d]
Index: []

In [4]: pd.read_sas("zero_observations.sas7bdat")

In [5]: 
```

This is `a.csv`:
```
$ cat zero_observations.csv
a,b,c,d
```

#### Problem description

When reading a SAS file with 0 records, it gives `None`. But when reading a CSV file with 0 records it gives an empty dataframe.

In SAS, we can atleast identify the datatypes correctly and make an empty dataframe with this.

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

<details>
INSTALLED VERSIONS
------------------
commit: None
python: 3.5.2.final.0
python-bits: 64
OS: Darwin
OS-release: 16.7.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: en_US.UTF-8
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.21.0
pytest: 3.2.1
pip: 9.0.1
setuptools: 23.0.0
Cython: 0.27.3
numpy: 1.13.3
scipy: 0.18.1
pyarrow: None
xarray: None
IPython: 6.1.0
sphinx: 1.4.9
patsy: None
dateutil: 2.6.1
pytz: 2017.3
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 2.0.0
openpyxl: None
xlrd: 1.1.0
xlwt: None
xlsxwriter: 0.9.8
lxml: None
bs4: None
html5lib: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.8.1
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None


</details>
