[7.x] Fix whereNull/whereNotNull for json in MySQL

[LastDragon-ru]

This PR fixes #31473, please see issue for more details, in short:

set @a='{"value":null}';

select
-- Actual behavior whereNull/whereNotNull
json_unquote(json_extract(@a, '$."value"')) is null,     -- false but should be true because value = null
json_unquote(json_extract(@a, '$."value"')) is not null, -- true  but should be false 

-- Expected
(json_extract(@a, '$."value"') is null     OR  json_type(json_extract(@a, '$."value"')) = 'NULL'),  -- true, ok
(json_extract(@a, '$."value"') is not null AND json_type(json_extract(@a, '$."value"')) != 'NULL')  -- false, ok
;

PS: I tested it only for MySQL, probably would be good also test for MariaDB and Percona.

[BSN4]

whereJsonLength works for me

//where not null
->whereJsonLength('json', '>=', 1); 

//where  null
->whereJsonLength('json', '<=', 0); 

[LastDragon-ru]

whereJsonLength works for me

@if4lcon is it work if key does not exist? Seems no? In any case whereNull much more readable for me...

[taylorotwell]

@staudenmeir do you have any feedback on this?

[staudenmeir]

Looks good to me. Great tests!

Changes made.

[GrahamCampbell]

Missing phpdoc.

[GrahamCampbell]

Same

[taylorotwell]

Fixed PHP doc issues.