why system/connect web services return nothing in drupal 7?
The services module allows a resource to define the following "things":


Operations: provide 5 different verbs


retrieve: Make a GET request based on the resource name and an identifier i.e. GET /endpoint/user/ID
index: Make a GET request based on the resource name to get a list of resources i.e. GET /endpoint/user
update: Make a PUT request to a resource name and an identifier i.e. PUT /endpoint/user/ID
create: Make an idempotent POST request to a resource name to create a new resource i.e. POST /endpoint/user
delete: Make a DELETE request to remove a resource by identifier i.e. DELETE /endpoint/user/ID

Actions: Allows to make a custom POST request to a resource. This is used for login, logout, etc... These are never GET.
Targeted Actions: Allows to make a custom POST request to a resource + identifier. This is used for making hierarchical resources.
Relationships: Allows to make a custom GET request to a resource + identifier. This is used for make hierarchical requests for things like group membership.


The system/connect resource is defined as an "action" above. This means that you must make a POST request instead of a GET request. And it may require a CSRF Token.

POST /login_x/system/connect HTTP/1.1
Content-type: application/json
Accept: application/json
X-CSRF-Token: 'TOKEN'