Stasis: Ensure transfer messages follow validation rules

This makes Stasis() event generation for transfer messages follow
validation rules. Currently, ast_json_null() is being used in place of
omitting a key entirely which falls afoul of these validation rules.

https://reviewboard.asterisk.org/r/3892/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@420408 f38db490-d61c-443f-a65b-d21fe96a405b