Bug 18933: Allow OPAC messaging SMS number to be set to empty

Bug 18933: Allow OPAC messaging SMS number to be set to empty

To test:
1) Ensure EnhancedMessagingPreferences is enabled and
EnhancedMessagingPreferencesOPAC is set to Show. Set SMSSendDriver to
Email
2) Log in to OPAC, go to your messaging tab
3) Set an SMS number if you havent already
4) Try to remove it (set to empty)
5) Notice that it says the number has been updated, but it has not
been set to empty
6) Apply patch and refresh page
7) Attempt to set the number to empty
8) Confirm this is successful

Sponsored-by: Catalyst IT
Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>