Allow to pass nil to client_id arg of device_credentials

[shibayu36]

closes #319

Changes

I changed to allow to pass nil to client_id arg of device_credentials, related to discussions in #319

We can use device_credentials method like the following

client.device_credentials # Fetch all device_credentials on the specified tenant
client.device_credentials('client_id') # Fetch device_credentials related to specified client_id
client.device_credentials(nil, user_id: 'user_id') # Fetch device_credentials related to specified user_id

I considered to change the interface like Auth0::Api::V2::Grants#grants, but it will break backwards compatibility, and it is the refactoring issue. So I only changed to allow client_id to receive nil.

References

#319

Testing

Please describe how this can be tested by reviewers. Be specific about anything not tested and reasons why. If this library has unit and/or integration testing, tests should be added for new functionality and existing tests should complete without errors.

• This change adds unit test coverage
• This change adds integration test coverage
• This change has been tested on the latest version of Ruby

Checklist

• I have read the Auth0 general contribution guidelines
• I have read the Auth0 Code of Conduct
• All existing and new tests complete without errors
• Rubocop passes on all added/modified files
  • I found some offenses by rubocop in lib/auth0/api/v2/device_credentials.rb and spec/lib/auth0/api/v2/device_credentials_spec.rb. But I left it at that because there were originally many offenses
• All active GitHub checks have passed

[stevehobbsdev]

This looks good - thanks again for your contribution!