Min/max does not work for dates with timezones if there are missing values in the data frame
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd

df = pd.concat([
  pd.date_range(start='1/1/2018', end='1/30/2018').to_series().dt.tz_localize("UTC"),
  pd.date_range(start='1/1/2018', end='1/20/2018').to_series().dt.tz_localize("UTC")
], axis=1)

df.min(axis=1)
```
#### Problem description

Without `tz_localize`, everything works as expected. Also, if both columns have the same length and time zones, it works just fine. However, the combination of time zones and different lengths results in:

```
2018-01-01   NaN
2018-01-02   NaN
2018-01-03   NaN
2018-01-04   NaN
2018-01-05   NaN
2018-01-06   NaN
2018-01-07   NaN
2018-01-08   NaN
2018-01-09   NaN
2018-01-10   NaN
2018-01-11   NaN
...
```
of type float64.

#### Expected Output

List of dates

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

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.6.5.final.0
python-bits      : 64
OS               : Linux
OS-release       : 4.4.0-17134-Microsoft
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : C.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 0.25.0
numpy            : 1.17.0
pytz             : 2019.1
dateutil         : 2.8.0
pip              : 19.1.1
setuptools       : 41.0.1
Cython           : 0.29.10
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : 1.1.8
lxml.etree       : 4.3.4
html5lib         : None
pymysql          : None
psycopg2         : 2.8.2 (dt dec pq3 ext lo64)
jinja2           : 2.10.1
IPython          : 7.5.0
pandas_datareader: None
bs4              : None
bottleneck       : 1.2.1
fastparquet      : None
gcsfs            : None
lxml.etree       : 4.3.4
matplotlib       : 3.1.1
numexpr          : 2.6.9
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
s3fs             : None
scipy            : 1.3.0
sqlalchemy       : 1.3.3
tables           : 3.5.2
xarray           : None
xlrd             : 1.2.0
xlwt             : None
xlsxwriter       : 1.1.8

</details>

I have found some related fixed issues, but not exactly this one.