app_page.c: Simplify dialplan using Page.

Dialplan has to be careful about passing an empty destination list or
empty positions in the list.  As a result, dialplan has to check for
these conditions before using Page.  Simplify dialplan by making Page
handle these conditions gracefully.

* Made tolerate empty positions in the paged device list.

* Reduced some warnings associated with the 's' option to verbose
messages.  The warning level for those messages really serves no purpose
as that is why the 's' option exists.

ASTERISK-28638

Change-Id: I95b64a6d6800cd1a25279c88889314ae60fc21e3