Bug 12361 - No default value for opaccolorstylesheet syspref

Bug 12361 - No default value for opaccolorstylesheet syspref

After install, the syspref opaccolorstylesheet will by default contain colors.css.
This file exist in OPAC prog theme but not in bootstrap theme.

This patch set the syspref empty by default in installer/data/mysql/sysprefs.sql.

Test plan :
- Create a fresh install
- Look at opaccolorstylesheet content
=> without this patch : is contains colors.css
=> with this patch : it is empty
- Enable opac bootstrap theme
- Go to opac main page
- Look at HTTP errors (with firebug for example)
=> without this patch : there is a 404 error because colors.css does not exist
=> with this patch : no such error

Signed-off-by: David Cook <dcook@prosentient.com.au>
Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>