Add JSON_LENGTH for MySQL

[MrMitch]

This adds support for the JSON_LENGTH function for MySQL.

https://dev.mysql.com/doc/refman/8.0/en/json-attribute-functions.html#function_json-length

[stevelacey]

Thanks for your contribution. I think when it's possible to mirror the SQL to the DQL exactly we probably should, see above amends.

[MrMitch]

Hi @stevelacey, thank you for your feedback. I've rebased the branch on master (since #355 was merged) and applied the changes you proposed (with a slight variation to prevent an error in SqlWalker::walkStringPrimary if $path is null).

[stevelacey]

@MrMitch thanks!