JSON nested_to_record Silently Drops Top-Level None Values
xref https://github.com/pandas-dev/pandas/pull/21164#issuecomment-394510095

`nested_to_record` is silently dropping `None` values that appear at the top of the JSON. This is IMO unexpected and undesirable.

#### Code Sample, a copy-pastable example if possible

```python
In [3]: data = {
   ...:     "id": None,
   ...:     "location": {
   ...:         "country": None
   ...:     }
   ...: }

In [5]: nested_to_record(data)
Out[5]: {'location.country': None}
```
#### Problem description

The top level `None` value should not be dropped but rather preserved along with lower levels for consistency.

#### Expected Output
```python
In [5]: nested_to_record(data)
Out[5]: {'id': None, 'location.country': None}
```

Note this will break a few tests in `pandas/test_normalize.py`

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

<details>

INSTALLED VERSIONS
------------------
commit: ab6aaf73a848a8725a23bb880be5221dd5ef5b3d
python: 3.6.4.final.0
python-bits: 64
OS: Darwin
OS-release: 17.5.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.24.0.dev0+67.gab6aaf73a
pytest: 3.4.1
pip: 10.0.1
setuptools: 38.5.1
Cython: 0.27.3
numpy: 1.14.1
scipy: 1.0.0
pyarrow: 0.8.0
xarray: 0.10.0
IPython: 6.2.1
sphinx: 1.7.0
patsy: 0.5.0
dateutil: 2.6.1
pytz: 2018.3
blosc: None
bottleneck: 1.2.1
tables: 3.4.2
numexpr: 2.6.4
feather: 0.4.0
matplotlib: 2.1.2
openpyxl: 2.5.0
xlrd: 1.1.0
xlwt: 1.3.0
xlsxwriter: 1.0.2
lxml: 4.1.1
bs4: 4.6.0
html5lib: 1.0.1
sqlalchemy: 1.2.5
pymysql: 0.8.0
psycopg2: 2.7.4 (dt dec pq3 ext lo64)
jinja2: 2.10
s3fs: 0.1.3
fastparquet: 0.1.4
pandas_gbq: 0.4.1
pandas_datareader: None

</details>
