LP#1669534 - OPAC hold request should not default to first SMS carrier

LP#1669534 - OPAC hold request should not default to first SMS carrier

Added empty <option> tag to address this and related form validation issues.

To test
-------
[0] Ensure that SMS messaging is enabled.
[1] Apply the patch and go to the place hold form in the public
    catalog. Verify that no mobile carrier is selected.
[2] Set the default SMS carrier and number for the user account
    and go back to the place hold form. Verify that the user's
    default carrier and number appear in the form.

Signed-off by: Cesar Velez <cesar.velez@equinoxinitiative.org>
Signed-off-by: Chris Sharp <csharp@georgialibraries.org>
Signed-off-by: Galen Charlton <gmc@equinoxinitiative.org>
Signed-off-by: Galen Charlton <gmc@equinoxinitiative.org>