[BUG] NPE Crash when paths is not provided

[agilob]

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Bounty to sponsor the fix (example)

Description

Exception in thread "main" java.lang.NullPointerException
        at org.openapitools.codegen.DefaultGenerator.processPaths(DefaultGenerator.java:1093)
        at org.openapitools.codegen.DefaultGenerator.generateApis(DefaultGenerator.java:556)
        at org.openapitools.codegen.DefaultGenerator.generate(DefaultGenerator.java:1008)
        at org.openapitools.codegen.cmd.Generate.execute(Generate.java:431)
        at org.openapitools.codegen.cmd.OpenApiGeneratorCommand.run(OpenApiGeneratorCommand.java:32)
        at org.openapitools.codegen.OpenAPIGenerator.main(OpenAPIGenerator.java:61)

openapi-generator version

4.3.1

OpenAPI declaration file content or url

openapi: 3.0.0
info:
  version: 1.0.0
  title: Whatever this is
  description: whatever this description is

servers:
  - url: http://127.0.0.1:8080/{basePath}
    variables:
      basePath:
        default: v1

components:
  schemas:
    MyModel:
      type: object
      description: Description of a model
      properties:
        sortOrder:
          type: string
          enum: [asc, desc]

Command line used for generation

openapi-generator generate -i openapi-spec.yaml -g dart --skip-validate-spec -o './'

I specifically do not want to generate controllers for this model

[auto-labeler]

👍 Thanks for opening this issue!
🏷 I have applied any labels matching special text in your issue.

The team will review the labels and make any necessary changes.