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/
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Merged revisions 422664 from http://svn.asterisk.org/svn/asterisk/branches/12
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Merged revisions 422665 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@422683 f38db490-d61c-443f-a65b-d21fe96a405b