ARI: allow json content type with zero length body

When a request was received with a Content-type of json,
the body was sent for json parsing - even if it was zero
length.  This resulted in ARI requests failing that were
valid, such as a channel DELETE with no parameters.  The
code has now been changed to skip json parsing with zero
content length.

(closes issue SWP-6748)
Reported by: Samuel Galarneau
Review: https://reviewboard.asterisk.org/r/3360/



git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@410858 f38db490-d61c-443f-a65b-d21fe96a405b