Bug 11207: fix issue where SMS number couldn't be cleared when editing patron

Bug 11207: fix issue where SMS number couldn't be cleared when editing patron

Test plan:
1/ edit a patron
2/ set a sms alert number
3/ save
4/ edit the patron
5/ set the sms alert number to a blank value
6/ save
7/ the sms number is now an empty string

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Passes all tests and QA script.

Test plan used:
- Activate the SMS functionality by giving a 'fake'
  SMSSendDriver of TEST
- Activate EnhancedMessagingPreferences
- Create a new patron, set Other phone to 'test'
- Notice that Other phone was copied to sms number
- Edit messaging preference section using the edit link
  below on the details tab and delete SMS number
- Verify it's not possible to unset the SMS number
- Apply patch
- Repeat deleting - this time it works

Signed-off-by: Galen Charlton <gmc@esilibrary.com>
(cherry picked from commit b15f25d8c672d198cd074fd16f01e1f67fef955e)
Signed-off-by: Fridolin SOMERS <fridolin.somers@biblibre.com>