Adding 204 HTTP code response to swagger and example. (Azure#12291)

Author: sofisa16

specification/cost-management/resource-manager/Microsoft.CostManagement/stable/2019-11-01/costmanagement.json

@@ -60,6 +60,9 @@
               "$ref": "#/definitions/ViewListResult"
             }
           },
+          "204": {
+            "description": "An empty response is sent when there is no information available within the selected scope."
+          },
           "default": {
             "description": "Error response describing why the operation failed.",
             "schema": {

specification/cost-management/resource-manager/Microsoft.CostManagement/stable/2019-11-01/examples/PrivateViewList.json

@@ -128,6 +128,7 @@
           }
         ]
       }
-    }
+    },
+    "204": {}
   }
 }