Inconsistent dtype of category in empty Series between dict and list input
#### Code Sample, a copy-pastable example if possible

```python
In [2]: pd.Series([], dtype='category')
Out[2]: 
Series([], dtype: category
Categories (0, object): [])

In [3]: pd.Series({}, dtype='category')
Out[3]: 
Series([], dtype: category
Categories (0, float64): [])

In [4]: pd.Series(dtype='category')
Out[4]: 
Series([], dtype: category
Categories (0, float64): [])
```
#### Problem description

The difference is unjustified.

#### Expected Output

The same. Probably ``Out[4]:``, which is also (implicitly) [tested](https://github.com/pandas-dev/pandas/blob/master/pandas/tests/series/test_combine_concat.py#L184).

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.5.3.final.0
python-bits: 64
OS: Linux
OS-release: 4.9.0-4-amd64
machine: x86_64
processor: 
byteorder: little
LC_ALL: None
LANG: en_GB.UTF-8
LOCALE: en_GB.UTF-8

pandas: 0.22.0.dev0+241.gf745e52e1
pytest: 3.0.6
pip: 9.0.1
setuptools: 33.1.1
Cython: 0.25.2
numpy: 1.12.1
scipy: 0.18.1
pyarrow: None
xarray: None
IPython: 5.2.2
sphinx: None
patsy: 0.4.1+dev
dateutil: 2.6.0
pytz: 2016.10
blosc: None
bottleneck: 1.2.0
tables: 3.3.0
numexpr: 2.6.1
feather: 0.3.1
matplotlib: 2.0.0
openpyxl: 2.3.0
xlrd: 1.0.0
xlwt: 1.2.0
xlsxwriter: None
lxml: 3.7.1
bs4: 4.5.3
html5lib: 0.999999999
sqlalchemy: 1.0.15
pymysql: None
psycopg2: None
jinja2: 2.8
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None


</details>
