fix(jsonapi): return empty data array for empty relation (#6270)

Author: GwendolenLynch

features/http_cache/tag_collector_service.feature

@@ -107,6 +107,9 @@ Feature: Cache invalidation through HTTP Cache tags (custom TagCollector service
                         "type": "RelatedDummy",
                         "id": "/related_dummies/1"
                     }
+                },
+                "related": {
+                    "data": []
                 }
             }
         }

features/jsonapi/absolute_url.feature

@@ -56,6 +56,11 @@ Feature: IRI should contain Absolute URL
         "type": "AbsoluteUrlRelationDummy",
         "attributes": {
           "_id": 2
+        },
+        "relationships": {
+          "absoluteUrlDummies": {
+            "data": []
+          }
         }
       }
     }

features/jsonapi/jsonapi.feature

@@ -205,6 +205,9 @@ Feature: JSON API basic support
               "type": "ThirdLevel",
               "id": "/third_levels/1"
             }
+          },
+          "relatedToDummyFriend": {
+            "data": []
           }
         }
       }

features/jsonapi/network_path.feature

@@ -56,6 +56,11 @@ Feature: IRI should contain network path
         "type": "NetworkPathRelationDummy",
         "attributes": {
           "_id": 2
+        },
+        "relationships": {
+          "networkPathDummies": {
+            "data": []
+          }
         }
       }
     }