Thanks, Colin
Writing if condition by the building target worked!
if $BUILDKITE_PULL_REQUEST; then
/opt/sonar-scanner/bin/sonar-scanner -Dsonar.login=$SONARCLOUD_TOKEN -Dsonar.pullrequest.provider=GitHub -Dsonar.pullrequest.key=$BUILDKITE_PULL_REQUEST
else
/opt/sonar-scanner/bin/sonar-scanner -Dsonar.login=$SONARCLOUD_TOKEN -Dsonar.pullrequest.provider=GitHub -Dsonar.branch.name=$BUILDKITE_BRANCH
fi