JSON_OBJECT accepts a NULL key and invents an empty-string key, where the documentation says an error occurs

JSON_OBJECT is documented to raise an error when a key name is NULL. It
does not: the key becomes the empty string, silently.

SELECT JSON_OBJECT(NULL, 1) AS v;

v

{"": 1}

No error, no warning, no note. The other half of the same documented
sentence is implemented, so the sentence is not dead text:

SELECT JSON_OBJECT('a');

ERROR 42000: Incorrect parameter count in the call to native

function 'JSON_OBJECT'

How to repeat

SELECT JSON_OBJECT(NULL, 1) AS literal_null;

CREATE TABLE tk (k VARCHAR(10), v INT);

INSERT INTO tk VALUES ('a',1),(NULL,2);

SELECT JSON_OBJECT(k, v) AS from_a_column FROM tk;

from_a_column

{"a": 1}

{"": 2}

DROP TABLE tk;

Root cause

The key argument is written into the object without being tested for
NULL, so `val_str()` returning NULL is composed as a zero-length key.

Contract, `scripts/fill_help_tables.sql:451` (topic 377): *"An error
will occur if there are an odd number of arguments, or any key name is
NULL."*

Why it matters

A row's identifying value is silently replaced by the empty string, and
two rows with NULL keys collide on one key name inside the same object,
so one of them is lost.

Affected versions

JSON_OBJECT was added in 10.2.3.

Verified on 10.11 only (10.11.19-MariaDB-debug, commit 1dab253482d).
Other branches were not tested.

Note

Found while auditing the JSON functions against their documented
contracts. Pinned as the deviation:
func_json_invalid.result:396-398 records JSON_OBJECT(NULL, 1) ->

. Nothing anywhere uses --error for it.