add missing check for root URLs length (#53239) (#53251)

(cherry picked from commit 46b7ca12e1)

Co-authored-by: Will Browne <wbrowne@users.noreply.github.com>

packages/grafana-toolkit/src/cli/tasks/plugin.ci.ts

@@ -140,7 +140,7 @@ const packagePluginRunner: TaskRunner<PluginCIOptions> = async ({ signatureType,
     if (signatureType) {
       manifest.signatureType = signatureType;
     }
-    if (rootUrls) {
+    if (rootUrls && rootUrls.length > 0) {
       rootUrls.forEach(assertRootUrlIsValid);
       manifest.rootUrls = rootUrls;
     }