regression: ts64->int64
#### Code Sample, a copy-pastable example if possible

```python
!pip install pandas==1.0.0
import datetime
import pandas as pd
pd.Series([datetime.datetime.now(), datetime.datetime.now(), None]).astype('int64')
```

=>
```
ValueError: Cannot convert NaT values to integer
```

#### Problem description

In 0.25.*, the above ran without an exception, while in 1.0.0, it now throws an exception. While the new behavior may be OK, it would be better as first a deprecation warning with a suggested  migration. 

(We had the same code for at least 1-2 years w/out issue. Still unclear on the proper fix for getting the int64 or int32 epoch out in a way that is friendly for both pre/post 1.0.0.)

#### Expected Output

Same behavior as 0.25.*:
```
0    1580769049934654000
1    1580769049934659000
2   -9223372036854775808
dtype: int64
```

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

<details>

/usr/local/lib/python3.6/dist-packages/psycopg2/__init__.py:144: UserWarning: The psycopg2 wheel package will be renamed from release 2.8; in order to keep installing from binary please use "pip install psycopg2-binary" instead. For details see: <http://initd.org/psycopg/docs/install.html#binary-install-from-pypi>.
  """)
/usr/local/lib/python3.6/dist-packages/pandas_datareader/compat/__init__.py:18: FutureWarning: pandas.util.testing is deprecated. Use the functions in the public API at pandas.testing instead.
  from pandas.util.testing import assert_frame_equal

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.6.9.final.0
python-bits      : 64
OS               : Linux
OS-release       : 4.14.137+
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.0.0
numpy            : 1.17.5
pytz             : 2018.9
dateutil         : 2.6.1
pip              : 19.3.1
setuptools       : 45.1.0
Cython           : 0.29.14
pytest           : 3.6.4
hypothesis       : None
sphinx           : 1.8.5
blosc            : None
feather          : 0.4.0
xlsxwriter       : None
lxml.etree       : 4.2.6
html5lib         : 1.0.1
pymysql          : None
psycopg2         : 2.7.6.1 (dt dec pq3 ext lo64)
jinja2           : 2.11.1
IPython          : 5.5.0
pandas_datareader: None
bs4              : 4.6.3
bottleneck       : 1.3.1
fastparquet      : None
gcsfs            : None
lxml.etree       : 4.2.6
matplotlib       : 3.1.2
numexpr          : 2.7.1
odfpy            : None
openpyxl         : 2.5.9
pandas_gbq       : 0.11.0
pyarrow          : 0.14.1
pytables         : None
pytest           : 3.6.4
pyxlsb           : None
s3fs             : 0.4.0
scipy            : 1.4.1
sqlalchemy       : 1.3.13
tables           : 3.4.4
tabulate         : 0.8.6
xarray           : 0.14.1
xlrd             : 1.1.0
xlwt             : 1.3.0
xlsxwriter       : None
numba            : 0.47.0
</details>
