Bug 14683: Unable to clear SMS number

Bug 14683: Unable to clear SMS number

Enables to clear SMS number.

To test:
1. Go to opac-messaging.pl
2. Insert SMS number and submit
3. Clear SMS number and submit
4. Observe that the sms number did not change
5. Apply patch
6. Clear SMS number and submit
7. Observe that the sms number changes

Sponsored-by: Vaara-kirjastot
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Adding a follow-up.

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>