added model with nullable enum example in test definitions

Author: HugoMario

modules/swagger-codegen/src/test/resources/3_0_0/petstore-with-composed-schemas.yaml

@@ -954,6 +954,17 @@ components:
         prefix: val
         name: member
       type: object
+    NullableEnumModel:
+      type: object
+      properties:
+        enumProp:
+          type: string
+          nullable: true
+          enum:
+            - a
+            - b
+            - null
+
 
 
   requestBodies:

modules/swagger-codegen/src/test/resources/3_0_0/petstore-with-fake-endpoints-models-for-testing.yaml

@@ -1813,6 +1813,17 @@ components:
           id: 1
         status: "available"
 
+  NullableEnumModel:
+    type: object
+    properties:
+      enumProp:
+        type: string
+        nullable: true
+        enum:
+          - a
+          - b
+          - null
+
     SinglePetResponse:
       type: object
       properties:

modules/swagger-codegen/src/test/resources/3_0_0/petstore.yaml

@@ -725,6 +725,16 @@ components:
     Test:
       type: string
       example: ""
+    NullableEnumModel:
+      type: object
+      properties:
+        enumProp:
+          type: string
+          nullable: true
+          enum:
+            - a
+            - b
+            - null
   requestBodies:
     Pet:
       content: