Bug 17324: Deal with branchcode NULL issue

Bug 17324: Deal with branchcode NULL issue

In a database update we alter the letter table, making a column
(branchcode) not be able to contain NULL values.
However if it already does, this SQL fails. So before changing the
column we change the NULL values

[Tue Sep 20 19:08:10 2016] updatedatabase.pl: DBD::mysql::db do failed:
Invalid use of NULL value [for Statement "
[Tue Sep 20 19:08:10 2016] updatedatabase.pl: ALTER TABLE letter MODIFY
COLUMN branchcode varchar(10) NOT NULL DEFAULT ''
[Tue Sep 20 19:08:10 2016] updatedatabase.pl: "] at
/home/mtompset/kohaclone/installer/data/mysql/updatedatabase.pl line
12638.

Reproduce:
on an ubuntu 16.04 LTS machine with MySQL 5.7 install git koha
development environment.
...
git checkout -b my_3.18.x origin/3.18.x
drop database koha_library;
create database koha_library;
quit;
web install all sample data
(ignore some issues with sample data, and one with mandatory)

git checkout -b bug_17234 origin/master
git bz apply 17292
git bz apply 17320
git bz apply 17260
git bz apply 17234
web install
Two warning messages are expected output from upgrade process, but
the above message is not.

Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Did not follow the test plan, but the changes make sense

Signed-off-by: Brendan Gallagher <brendan@bywatersolutions.com>