Update typesystem.rs, added MYSQL_TYPE_NULL

[luca-trigili]

No description provided.

[wangxiaoying]

Hi @luca-trigili , thanks for the PR!

Can you also add a test for the null type in our CI? You can refer to this commit: Amar1729@3391f71 which adds a new type to postgres. Basically you need to add a new test type null to mysql.sql (new column in create table, insert values) and then modify corresponding tests (here and here) in test_mysql.py by checking the result of this new type.