RT_TRIGGER_URL causes failure in pipeline if build triggered manually

[stewcee]

We've added the Artifactory trigger configuration to our Jenkinsfile and it is working as expected when triggered by a change in Artifactory

We followed this example:
https://github.com/jfrog/project-examples/blob/master/jenkins-examples/pipeline-examples/declarative-examples/build-trigger-example/Jenkinsfile

The comments above RT_TRIGGER_URL mention that the value may be empty if the build isn't triggered by a change in Artifactory, however if a user tries to manually trigger this pipeline job, we get the following error:
Also: org.jenkinsci.plugins.workflow.actions.ErrorAction$ErrorId: f8fe75f7-d762-4b5a-999c-b25b1da78862 java.lang.IllegalArgumentException: One or more variables have some issues with their values: RT_TRIGGER_URL

Is there a way to ignore RT_TRIGGER_URL being empty if the build is triggered manually instead of by the Artifactory trigger?