* parser: regard NULL as literal when normalizing queries
Currently, Normalize() converts "SELECT NULL" to "select null", while NULL is
regarded as a literal in MySQL, as described in the documentation.
https://dev.mysql.com/doc/refman/8.0/en/literals.html
It seems natural that Normalize() would convert "SELECT NULL" to "select ?"
from the consistency viewpoint.
* Handle case of 'select `null`'
* Fix quotation
Co-authored-by: Arenatlx <ailinsilence4@gmail.com>
Co-authored-by: kennytm <kennytm@gmail.com>
