Bug 16402: Make kohastructure.sql valid for MySQL v5.7

Bug 16402: Make kohastructure.sql valid for MySQL v5.7

A primary key should not contain null value, and MySQL v5.7 is now more
strict and won't accept our errors in the DB structure.

Test plan:
  mysql < installer/data/mysql/kohastructure.sql
should work using MySQL v5.7

QA:
- letter.branchcode should not be NULL, the default (all) is an empty
string
- permissions.code should not be NULL, it's always a string

Signed-off-by: Bernardo Gonzalez Kriegel <bgkriegel@gmail.com>
Clean install of Ubuntu 16.05, MySQL 5.7.11
kohastructure.sql loads ok, just one warning
No errors

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>