APT repository install fails when mysql root user has password

When installing Xwiki from APT repository on Ubuntu release Trusty (14.04LTS, 64-bit) the installation fails when mysql root user has password.

Below a piece of the installation output, where things go wrong:

...
Creating config file /etc/logrotate.d/tomcat7 with new version

• Starting Tomcat servlet engine tomcat7 [ OK ]
  Setting up xwiki-enterprise-common (6.0) ...
  Setting up xwiki-enterprise-mysql-common (6.0) ...
  dbconfig-common: writing config to /etc/dbconfig-common/xwiki.conf

Creating config file /etc/dbconfig-common/xwiki.conf with new version

Creating config file /etc/xwiki/hibernate.cfg.xml with new version
granting access to database xwiki for xwiki@localhost: success.
verifying access for xwiki@localhost: success.
creating database xwiki: success.
verifying database xwiki exists: success.
dbconfig-common: flushing administrative password
ERROR 1045 (28000): Access denied for user 'root'@'localhost' (using password: NO)
dpkg: error processing package xwiki-enterprise-mysql-common (--configure):
subprocess installed post-installation script returned error exit status 1
Setting up xwiki-enterprise-tomcat-common (6.0) ...
dpkg: dependency problems prevent configuration of xwiki-enterprise-tomcat7-mysql:
xwiki-enterprise-tomcat7-mysql depends on xwiki-enterprise-mysql-common (= 6.0); however:
Package xwiki-enterprise-mysql-common is not configured yet.

dpkg: error processing package xwiki-enterprise-tomcat7-mysql (--configure):
dependency problems - leaving unconfigured
...

Finally apt-get install end with these messages:

Processing triggers for libc-bin (2.19-0ubuntu6) ...
Processing triggers for ureadahead (0.100.0-16) ...
Processing triggers for ca-certificates (20130906ubuntu2) ...
Updating certificates in /etc/ssl/certs... 0 added, 0 removed; done.
Running hooks in /etc/ca-certificates/update.d....
done.
done.
Errors were encountered while processing:
xwiki-enterprise-mysql-common
xwiki-enterprise-tomcat7-mysql
E: Sub-process /usr/bin/dpkg returned an error code (1)

The solution is to remove the root user password like this:
mysqladmin -u root -pCURRENTPASSWORD password ''

as mentioned in
http://platform.xwiki.org/xwiki/bin/view/AdminGuide/InstallationViaAPT

Expected behaviour:
I expect the debconf installer to correctly apply the supplied mysql root user password while installing and granting xwiki user tables.