Sum of an empty Series is now nan, should be 0 (regression in 0.21 from 0.20.3)
#### Code Sample, a copy-pastable example if possible

Current (`0.21`) **incorrect** behavior:

```python
>>> import pandas as pd
>>> pd.__version__
u'0.21.0'
>>> pd.Series().sum()
nan
```

Old (`0.20.3`) **correct** behavior:

```python
>>> import pandas as pd
>>> pd.__version__
u'0.20.3'
>>> pd.Series().sum()
0
```

#### Problem description

Sum of an empty series should be `0`, not `nan`, because otherwise the following invariant is violated:
```
pd.concat([s1,s2]).sum() == s1.sum() + s2.sum()
```

#### Expected Output

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

<details>
INSTALLED VERSIONS
------------------
commit: None
python: 2.7.14.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: C
LOCALE: None.None

pandas: 0.21.0
pytest: None
pip: 9.0.1
setuptools: 36.6.0
Cython: None
numpy: 1.13.3
scipy: 1.0.0
pyarrow: None
xarray: None
IPython: 5.5.0
sphinx: None
patsy: None
dateutil: 2.6.1
pytz: 2017.3
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 2.1.0
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: 1.0.2
lxml: None
bs4: None
html5lib: 1.0b10
sqlalchemy: 1.1.15
pymysql: None
psycopg2: 2.7.3.2 (dt dec pq3 ext lo64)
jinja2: 2.9.6
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
</details>
