Bug 23901: Fix sms_input is null in opac-messaging.tt

Bug 23901: Fix sms_input is null in opac-messaging.tt

When sms messaging is disabled, JavaScript breaks in opac-messaging.tt.
Bug introduced in Bug 22862.

To test:
1. Enable EnhancedMessagingPreferences system preference
2. Disable sms messaging by unsetting SMSSendDriver system preference
3. Go to OPAC -> your messaging
4. Observe JavaScript error "sms_input is null" in your browser console
5. Apply patch
6. Refresh page and observe the error is gone

Sponsored-by: Koha-Suomi Oy
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>