APIv4 - Use empty string instead of 'null' to pass null values to the database

APIv4 - Use empty string instead of 'null' to pass null values to the database

`CRM_Core_DAO::copyValues()` already converts '' to 'null', and passing the string 'null'
into `BAO::writeRecords` or `BAO::create` functions prematurely can introduce subtle bugs
because the string 'null' is `!empty()` whereas '' is `empty()`.