In a multibranch pipeline job we see that GIT_URL is set in builds for branches, but for PR builds it is null. However, GIT_COMMIT is set in both cases.
We're using bitbucket-branch-source-plugin for the multibranch part but I believe it's git-plugin that sets the GIT_URL env variable.
We are also using declarative pipelines, with implicit checkout.

It should be reproducable with a simple pipeline like this:

pipeline {
    agent any
    stages {
        stage('Test') {
            steps {
                echo "GIT_COMMIT: ${env.GIT_COMMIT}"
                echo "GIT_URL: ${env.GIT_URL}"
            }
        }
    }
}