BUG: objToJson.c - fix return value
- [x] closes #31463

https://dev.azure.com/pandas-dev/pandas/_build/results?buildId=27217&view=logs&j=3a03f79d-0b41-5610-1aa4-b4a014d0bc70&t=fe74a338-551b-5fbb-553d-25f48b1836e8&l=687

Seems like this warning is somehow causing an error in the users builds of pandas
Seems to not work on mac:

![image](https://user-images.githubusercontent.com/16733618/73523731-677c2000-4403-11ea-9c43-e51267cb36e1.png)
 Not at a computer to check but just might want to return NULL at the end of the function instead of changing it from a void pointer to void. I think original error is that the import_array() macro may only return that in some versions, so can force to work for all by returning NULL (though also a little strange) Thanks Will - confirmed we no longer see the below warnings with this change.
`control reaches end of non-void function`

https://dev.azure.com/pandas-dev/pandas/_build/results?buildId=27360&view=logs&j=3a03f79d-0b41-5610-1aa4-b4a014d0bc70&t=fe74a338-551b-5fbb-553d-25f48b1836e8

 thanks @alimcmaster1 ping on green (ex the 37 build which i just have to backport to CI). @jreback - All green ex 37. I can merge https://github.com/pandas-dev/pandas/pull/31573 into this branch thanks @alimcmaster1 