res_pjsip: When no global type the debug option defaults to "yes"

If the global section was not specified in pjsip.conf then the configuration
object does not exist in sorcery so when retrieving "debug" option it would
return NULL.  Then the NULL result was passed to ast_false utils function
which would return false because it wasn't set to some representation of
false, thus enabling sip debug logging.  Made it so if the global config object
does not exist then it will return a default of "no" for sip debugging.

(issue ASTERISK-23038)
Reported by: Rusty Newton



git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@407442 f38db490-d61c-443f-a65b-d21fe96a405b