APIv3 returns string "null" after calling "create" with empty string

When calling the "create" API, one receives an updated record as the response. However, the response is malformed because any field that contained an empty-string ('') is converted to string "null".

See example at:

https://github.com/civicrm/civihr/commit/030a14c1ba48b945ac4f3d4ce869129679460e28#L0R53

In the CiviHR Job UI, this has the effect of making it impossible to save and then re-save a form.