`deck file openapi2kong` panics if the file doesn't have paths field

[Prashansa-K]

Check: Kong/deck#1422

Test OAS file:

openapi: 3.0.0
info:
  title: httpbin
  version: "0.2"
  /get:
    get:
      operationId: /get
      responses:
        "200":
          description: "get: response 200"
  /post:
    post:
      operationId: /post
      responses:
        "200":
          description: "post: response 200"
servers:
  - url: https://httpbin.org