Mariadb 10.2.7 null values change in information schema, #2817

[belgattitude]

Since MariaDB 10.2.7, information_schema.columns now stores 'null' in column_default instead of null. MySQL 5.7 is not affected.

This P/R should fix the two doctrine-related issues:

• Comparison Problem Default 'NULL' #2817 (invalid detection of column default values)
• MariaDB 10.2.7 BC break orm#6565 (schema update will alter all tables with columns_default='null')

Not very used to doctrine, so please check and correct me if I'm wrong somewhere.

(see https://jira.mariadb.org/browse/MDEV-13132 and https://jira.mariadb.org/browse/MDEV-13341)

[belgattitude]

Sorry messed up with my clone (forgot to create a branch), closing and will reopen a new one in few minutes.