Fix Segfault In features-config.c When Application Has No Arguments

Some applications do not require arguments.  Therefore, when parsing application
maps in features.conf, it is possible that app_data will be set to NULL.

* This patch sets app_data to "" if it is NULL.

Review: https://reviewboard.asterisk.org/r/2804
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Merged revisions 399294 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@399295 f38db490-d61c-443f-a65b-d21fe96a405b