MISP: Improper sharing group authorization check when adding attributes

An improper authorization check in MISP’s attribute creation endpoint allowed an authenticated user with permission to add attributes to submit a sharing_group_id without triggering the corresponding sharing group authorization check, as long as the attribute distribution value was not explicitly set to 4 — “sharing group”. As a result, a user could reference or associate an attribute with a sharing group they were not authorized to use. This could lead to an access-control bypass affecting the integrity of attribute sharing metadata and potentially expose or misuse restricted sharing group relationships. The patch changes the authorization logic so that the sharing group permission check is performed whenever a non-empty sharing_group_id is provided, regardless of the selected distribution value.