Bug 16813: Allow empty string for OPACBaseURL

Bug 16813: Allow empty string for OPACBaseURL

The pref is prefixed by 'http://', but it should not when the pref is
set to an empty string.
This will fix the bug raised on bug 14790 comment 14.

Test plan:
Empty OPACBaseURL and confirm that it's save as it in DB

Signed-off-by: Hector Castro <hector.hecaxmmx@gmail.com>
OPACBaseURL is saved up empty. Prefix http:// is not saved anymore

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
(cherry picked from commit e9b6a5e31e1bc225a6e620dcefbd9545bb6d98bf)
Signed-off-by: Frédéric Demians <f.demians@tamil.fr>
(cherry picked from commit 4fa7e90b0e650cd5e9e33a708b89052238197f30)
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>