Cannot get Artifactory instance when default credentials are not set

[RecuencoJones]

Describe the bug

Artifactory.server('artifactory-server-id') throws NullPointerException if default deployer credentials are set to - none -.

To Reproduce

• From Jenkins configuration, go to JFrog section
• Tick Use the Credentials Plugin
• Add an Artifactory server instance with only the server url and an instance id (artifactory-server-id).
• Create a folder
• Create usernamePassword credentials within folder (some-folder-scoped-credentials)

• Attempt to use it in a scripted pipeline:

  def server = Artifactory.server('artifactory-server-id')
  
  server.credentialsId = 'some-folder-scoped-credentials'

Expected behavior

A new artifactory server instance using given folder credentials

Versions

• Jenkins Artifactory plugin version: 3.18.3
• Jenkins operating system: Ubuntu 18 (docker)
• Artifactory Version: 7.49.6 (Commercial)