Behaviour of concat with empty Series has changed between 0.18.0 and 0.18.1
#### Code Sample, a copy-pastable example if possible

``` python
import pandas as pd

s_empty = pd.Series()
s_filled = pd.Series([1, 2, 3, 4, 5])

pd.concat([s_empty, s_filled], 1)
```
#### Expected Output

Expect to be consistent between versions: previously would not add any columns for the empty `Series`, now a column of `NaN` is appended. (I can not see this mentioned in the what's new so think this is an unintended change.)

In `0.18.0`:

```
   0
0  1
1  2
2  3
3  4
4  5
```

In `0.18.1` (and tested on `0.18.1+25.g4aa6323`):

```
    0  1
0 NaN  1
1 NaN  2
2 NaN  3
3 NaN  4
4 NaN  5
```
#### output of `pd.show_versions()`

```
INSTALLED VERSIONS
------------------
commit: None
python: 2.7.11.final.0
python-bits: 64
OS: Darwin
OS-release: 15.4.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_GB.UTF-8

pandas: 0.18.1
nose: None
pip: 8.1.2
setuptools: 21.0.0
Cython: None
numpy: 1.11.0
scipy: 0.17.0
statsmodels: None
xarray: None
IPython: 4.2.0
sphinx: None
patsy: None
dateutil: 2.5.3
pytz: 2016.4
blosc: None
bottleneck: None
tables: None
numexpr: None
matplotlib: 1.5.1
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.8
boto: None
pandas_datareader: None
```

In my `0.18.0` environment the only differences in `show_versions` is the `pandas` entry and `pandas_datareader` is not present.
