res/res_sorcery_config: Prevent crash from misconfigured sorcery.conf

Misconfiguring sorcery.conf with a 'config' wizard with no extra data
will currently crash Asterisk on startup, as the wizard requires a comma
delineated list to parse. This patch updates res_sorcery_config to check
for the presence of the data before it starts manipulating it.

Change-Id: I4c97512e8258bc82abe190627a9206c28f5d3847