JSON_MERGE_PATCH raises no diagnostic for a malformed object argument merged onto SQL NULL, though the same characters are refused in every other position

An object argument merged onto SQL NULL is dropped, which is the right
answer - nothing of it reaches the result whether it is a document or
not. What is missing is any diagnostic: in this one argument position a
malformed document draws nothing, while the same function refuses the
same characters everywhere else.

SELECT JSON_MERGE_PATCH(NULL,'{"a":1 "b":2}','[1,2]') AS v;

v

[1, 2]

No warning. The same broken object in any other position is refused
with Warning 4038 naming its position.

How to repeat

SELECT JSON_MERGE_PATCH(NULL,'{"a":1 "b":2}','[1,2]')           AS v;

SELECT JSON_MERGE_PATCH(NULL,'{"a":1,','[1,2]')                 AS v;

SELECT JSON_MERGE_PATCH(NULL,'{','[1,2]')                       AS v;

SELECT JSON_MERGE_PATCH(NULL,'{"a":1 "b":2}','[1,2]','{"c":3}') AS v;

-- [1, 2] / [1, 2] / [1, 2] / {"c": 3}, all silent

The answers themselves are not in dispute: the two-argument forms are
NULL, and no partial content ever reaches the result.

Root cause

The arm that merges onto SQL NULL returns before the point where a
refusal would be reported, on the reasoning - stated in the code - that
an object merged onto NULL contributes nothing either way. That much is
true; the sentence that follows it is not, since the same characters are
in fact refused in every other argument position rather than spoken for.

Affected versions

JSON_MERGE_PATCH was added in 10.2.25.

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 correct:
func_json_merge_broken.result:113-116 records
JSON_MERGE_PATCH(NULL, '

', '[3,4]') -> [3, 4] with no
Warnings block at all.