fix: standardize nullable refs and correct array items syntax

Author: bliuchak

apify-api/openapi/components/schemas/actor-builds/Build.yaml

@@ -72,9 +72,9 @@ properties:
     type: string
     example: 0.1.1
   actorDefinition:
-      oneOf:
+      allOf:
         - $ref: "../actors/ActorDefinition.yaml"
-        - type: "null"
+      nullable: true
 example:
   id: HG7ML7M8z78YcAPEB
   actId: janedoe~my-actor

apify-api/openapi/components/schemas/users/Proxy.yaml

@@ -10,4 +10,4 @@ properties:
   groups:
     type: array
     items:
-      - $ref: ./ProxyGroup.yaml
+      $ref: ./ProxyGroup.yaml