Bug 21891: (follow-up) Update database to set blank values to "default"

Bug 21891: (follow-up) Update database to set blank values to "default"

This patch adds a database update which will update any blank values of
the XSLTDetailsDisplay system preference to "default." Although a blank
value will still result in the default XSLT view, it's more consistent
with the other XSLT preferences to use "default."

To test, apply the patch and set the XSLTDetailsDisplay to a blank
value. Run the database update. The value of the XSLTDetailsDisplay
system preference should now be "default."

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>