Bug 18755: Allow empty passwords in Patron Info to return OK

Bug 18755: Allow empty passwords in Patron Info to return OK

With this patch a parameter 'allow_empty_passwords="1" can be added to a
login in the SIP configuration file to allow the behaviour as was normal
before the patch for bug 16610 was applied. Some sip clients rely on
this behaviour sending an empty password field when they wish to
validate to user but do not have the password.
If a password is supplied it will be validated

A test has been added to Message.t to confirm this behaviour

Signed-off-by: Mark Tompsett <mtompset@hotmail.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>