Update of Rule 2 in papiNet-JSON-Style-Guide.md to specify all HTTP Status Codes to use.

[larsolofsson]

Proposed update of rule 2 to include all HTTP Status Codes to use. See also issue #287.

Rule 2 - HTTP Status Codes

The following HTTP Status Codes MUST be used for the specified cases below..

GET a list of items

• 200 OK, when items found.
• 200 OK with an empty collection, when no items found,
• 400 Bad request

GET details of an item

• 200 OK, when the item is found
• 404 Not Found, when the item is not found
• 400 Bad request

PUT to create an item

• 201 Created with no reponse body, when created successfully.
• 400 Bad request

PUT to replace an existing item

• 204 No Content with no response body, when replaced successfully.
• 400 Bad request

How to handle an empty collection
An empty collection MUST be returned with HTTP Status Code 200 OK. The HTTP response message body MUST contain the standard pagination envelope, where the total count is set to 0, and the collection array property is present and empty.

This ensures consistent response structures and simplifies client handling. The schema enforces this behavior by explicitly allowing an empty array (minItems: 0).