Call IDs: Fix appearance of call ID in core show channels when NULL

NULL call IDs were meant to appear as '(none)' but instead were showing
the contents of an uninitialized character buffer.

ASTERISK-24223
Review: https://reviewboard.asterisk.org/r/3979/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@422664 f38db490-d61c-443f-a65b-d21fe96a405b