Help text for four JSON mutators promises NULL for any NULL argument, but a NULL value stores a JSON null

This is a documentation defect, not a code defect. The behaviour is
the MySQL-compatible one and is what the test suite pins; four help rows
describe it wrongly.

SELECT JSON_SET('{}', '$.a', NULL)          AS s,

       JSON_REPLACE('{"a":1}', '$.a', NULL) AS r,

       JSON_ARRAY_APPEND('[1]', '$', NULL)  AS a,

       JSON_ARRAY_INSERT('[1]', '$[0]', NULL) AS i;

s             r             a          i

{"a": null}   {"a": null}   [1, null]  [null, 1]

All four help rows say "or NULL if any of the arguments are NULL". A
NULL in the value position does not make the result NULL; it stores a
JSON null, which is the useful and expected behaviour.

How to repeat

Read the help text beside the behaviour:

SELECT JSON_SET('{"a":1}','$.b',NULL,'$.c',2) AS v;

v

{"a": 1, "b": null, "c": 2}

A second false clause, on the JSON_SET row only

The JSON_SET row promises NULL for one further case the server does not
give it: "or the optional path fails to find an object". A path that
finds nothing leaves the document alone and hands it back; NULL comes
only from a NULL json_doc or a NULL path.

SELECT JSON_SET('{"a":1}', '$.b.c', 2) AS v;

v

{"a": 1}

SELECT JSON_SET('[1]', '$[0].x', 2) AS v;

v

[1]

For contrast, the two arguments that do produce NULL:

SELECT JSON_SET(NULL, '$.a', 1) AS v;

v

NULL

SELECT JSON_SET('{"a":1}', NULL, 1) AS v;

v

NULL

The wording that is right

JSON_INSERT's row (topic 368) is accurate and is the model the other
four should follow: *"returning the resulting document or NULL if either
of the json_doc or path arguments are null"*.

The four rows to correct in scripts/fill_help_tables.sql are topics 356
(JSON_ARRAY_APPEND), 357 (JSON_ARRAY_INSERT), 386 (JSON_REPLACE) and 389
(JSON_SET). Topic 389 needs the second clause removed as well as the
first corrected.

Affected versions

The four help rows have carried this wording since the functions
were documented.

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. The code behaviour is pinned and correct -
func_json_invalid.result:360-362 records

JSON_SET('{"a":1}', '$.a', NULL) -> {"a": null}.