jenkins: fix files diff on orphan branches (#33835)

diverge

Jenkinsfile

@@ -81,7 +81,7 @@ def deviceStage(String stageName, String deviceType, List extra_env, def steps)
 
     def extra = extra_env.collect { "export ${it}" }.join('\n');
     def branch = env.BRANCH_NAME ?: 'master';
-    def gitDiff = sh returnStdout: true, script: 'curl -s -H "Authorization: Bearer ${GITHUB_COMMENTS_TOKEN}" https://api.github.com/repos/commaai/openpilot/compare/master...${GIT_BRANCH} | jq .files[].filename', label: 'Getting changes'
+    def gitDiff = sh returnStdout: true, script: 'curl -s -H "Authorization: Bearer ${GITHUB_COMMENTS_TOKEN}" https://api.github.com/repos/commaai/openpilot/compare/master...${GIT_BRANCH} | jq .files[].filename || echo "/"', label: 'Getting changes'
 
     lock(resource: "", label: deviceType, inversePrecedence: true, variable: 'device_ip', quantity: 1, resourceSelectStrategy: 'random') {
       docker.image('ghcr.io/commaai/alpine-ssh').inside('--user=root') {