Today new version of JSON UDF functions: 0.3.1 was released. This is development release which contains new functionality. You can download functions from the MySQL Labs website.
What is new?
Default value for compile option `WITH_PCRE` now is `bundled` independently from the
platform (Bug #71265/18081332 Library name mismatch for PCRE on Ubuntu)
New values, passed to functions `JSON_APPEND`, `JSON_REPLACE`, `JSON_SET` as numbers, empty strings or `NULL`s are converted to JSON objects. Objects, passed as strings are not converted: you are still responsible to put quotes around string values. Also these functions check if element which needs to be inserted is valid JSON value. (