[Regression] to_json doesn't export Int64 dtype correctly
#### Code Sample

In 1.0.1:

```python
pd.Series([0,1,None,3], dtype="Int64").to_json()
>> '{"0":0,"1":1,"2":{},"3":3}'
```

#### Problem description

Not sure if that's the desired result, notice the object instead of null value. 

#### Expected Output

0.25.3 used to produce this:

```python
pd.Series([0,1,None,3], dtype="Int64").to_json()
>> # '{"0":0,"1":1,"2":null,"3":3}'
```

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

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.6.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 19.2.0
machine          : x86_64
processor        : i386
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : None.UTF-8

pandas           : 1.0.1
numpy            : 1.18.1
pytz             : 2019.3
dateutil         : 2.8.0
pip              : 20.0.2
setuptools       : 45.2.0.post20200209
Cython           : None
pytest           : 5.3.5
hypothesis       : None
sphinx           : 2.4.3
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : 4.4.1
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.11.1
IPython          : 7.12.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : 4.4.1
matplotlib       : 3.1.3
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : 0.16.0
pytables         : None
pytest           : 5.3.5
pyxlsb           : None
s3fs             : None
scipy            : 1.4.1
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : 1.2.0
xlwt             : None
xlsxwriter       : None
numba            : 0.48.0

</details>
