Fixing check order for empty body in get_body()

Cinder API logged the following debug message for each request with
empty body:
"Unrecognized Content-Type provided in request get_body"
Check order was fixed in get_body function to follow the correct flow
and display the right message (empty body).

Change-Id: Ie561983a4da791a412fea3fc390dab718dfce191
Closes-Bug: #1252692
Merge "Fixing check order for empty body in get_body()"