Building on @elouan-keryell-even 's answer, you can create a function in a shared library to wrap the check, like this:

@NonCPS
def getCurrentUsername() {
    String userId
    if(currentBuild.upstreamBuilds){
        userId = currentBuild.upstreamBuilds[0].getBuildCauses()[0].userId
    } else {
        userId = currentBuild.getBuildCauses()[0].userId
    }    
    return userId.toLowerCase().trim()
}

Then in your Jenkinsfile you can do:
getCurrentUsername()