test:suites:api: modified error message

In the "/* perform the same update again */" area the error
message should be "unable to update an non-empty object"
instead of "unable to update an empty object".

Signed-off-by: Andrei Epure <epure.andrei@gmail.com>
jansson:doc: modified incref doc

Signed-off-by: Andrei Epure <epure.andrei@gmail.com>