LP#1865533: save changes on Edit Hold in My Account

LP#1865533: save changes on Edit Hold in My Account

When editing a hold in the OPAC, changes are not saved if the SMS field
is blank.  The sms_notify value is being passed as an empty string, and
this non-null value violates the "sms_check" constraint on
action.hold_request.  Replacing the empty string with undef allows the
hold update to succeed.

Signed-off-by: Jeff Davis <jdavis@sitka.bclibraries.ca>
Signed-off-by: Terran McCanna <tmccanna@georgialibraries.org>
Signed-off-by: Chris Sharp <csharp@georgialibraries.org>