app_chanisavail.c: Simplify dialplan using ChanIsAvail.

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

* Made tolerate empty positions in the device list.

* Simplified the code and eliminated some unnecessary indention.

ASTERISK-28638

Change-Id: I9e4b67e2cbf26b2417c2d03485b8568e898931d3