Solaris doesn't like outputting a NULL to a %s in format strings.

Detect all platforms that don't like that, either, and ensure that when documentation is
missing, we pass a non-NULL pointer when outputting the corresponding documentation.

(closes issue #16689)
 Reported by: bklang
 Patches: 
       20100209__issue16689__with_tests.diff.txt uploaded by tilghman (license 14)
 
Review: https://reviewboard.asterisk.org/r/497/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@246030 65c4cc65-6c06-0410-ace0-fbb531ad65f3