Migrate tests to new format, update common ver, fix spec for nullables

spec/openapi.yaml

@@ -334,11 +334,14 @@ components:
       type: object
       properties:
         code:
-          type: string
+          oneOf:
+            - type: string
+            - type: "null"
           description: The country code.
         name:
-          type: string
-          description: The country name.
+          oneOf:
+            - type: string
+            - type: "null"
       example:
         code: CA
         name: Canada