refactor(openapi): replace user-defined null array type with nullable (#1752)

Replaces user-defined type EmptyNull with the
nullable defined by the OpenAPI specification.

openapi/SwarmCommon.yaml

@@ -93,9 +93,8 @@ components:
       type: object
       properties:
         lastcheques:
-          oneOf:
-            - $ref: '#/components/schemas/EmptyNull'
-            - type: array
+          type: array
+          nullable: true
           items:
             $ref: "#/components/schemas/ChequePeerResponse"
 
@@ -139,10 +138,6 @@ components:
       pattern: "^[A-Fa-f0-9]{40}$"
       example: "36b7efd913ca4cf880b8eeac5093fa27b0825906"
       
-    EmptyNull:
-      type: 'null'
-      description: When no entries 'null' is returned instead of empty array.
-
     FileName:
       type: string
 
@@ -214,9 +209,8 @@ components:
       type: object
       properties:
         tags:
-          oneOf:
-            - $ref: '#/components/schemas/EmptyNull'
-            - type: array
+          type: array
+          nullable: true
           items:
             $ref: "#/components/schemas/NewTagResponse"
 
@@ -228,9 +222,8 @@ components:
       type: object
       properties:
         peers:
-          oneOf:
-            - $ref: '#/components/schemas/EmptyNull'
-            - type: array
+          type: array
+          nullable: true
           items:
             $ref: "#/components/schemas/Address"
 
@@ -256,9 +249,10 @@ components:
       type: object
       properties:
         stamps:
-          oneOf:
-            - $ref: '#/components/schemas/EmptyNull'
-            - $ref: "#/components/schemas/PostageBatches"
+          type: array
+          nullable: true
+          items:
+            $ref: "#/components/schemas/PostageBatch"
 
     BatchIDResponse:
       type: object
@@ -294,12 +288,6 @@ components:
         utilization:
           type: integer
 
-    PostageBatches:
-      type: array
-      items:
-        $ref: "#/components/schemas/PostageBatch"
-
-
     Settlement:
       type: object
       properties:
@@ -318,9 +306,8 @@ components:
         totalSent:
           type: integer
         settlements:
-          oneOf:
-            - $ref: '#/components/schemas/EmptyNull'
-            - type: array
+          type: array
+          nullable: true
           items:
             $ref: "#/components/schemas/Settlement"
 
@@ -357,9 +344,8 @@ components:
       type: object
       properties:
         addresses:
-          oneOf:
-            - $ref: '#/components/schemas/EmptyNull'
-            - type: array
+          type: array
+          nullable: true
           items:
             $ref: "#/components/schemas/SwarmOnlyReference"