REST API: DB check, 204 when empty answer

[matleppa]

Description

When GET method is used and response contains only an empty list, HTTP code 204 must be used instead of 200.

Concerns at least following methods:
* GET /users , when list is empty
* GET /users/{id}, when not found
* GET users/updates, when list is empty
* GET /apis, when not found
* GET /organizations, when list is empty

[preriasusi]

It's done.