[Regression] to_json doesn't export Int64 dtype correctly

[buhrmann]

Code Sample

In 1.0.1:

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:

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

Output of pd.show_versions()

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

[WillAyd]

Should have already been fixed in #31748 if you want to try on master. Otherwise should be part of the 1.0.2 release

[buhrmann]

Ok, great. Sorry I didn't see this when searching for previous issues.