Update default scan execution yaml

- it now specified required fields name and branches

Changelog: changed

EE: true

ee/app/assets/javascripts/threat_monitoring/components/policy_editor/scan_execution_policy/lib/index.js

@@ -3,13 +3,13 @@ export { toYaml } from './to_yaml';
 export * from './humanize';
 
 export const DEFAULT_SCAN_EXECUTION_POLICY = `type: scan_execution_policy
-name: ''
+name: # name is mandatory
 description: ''
 enabled: false
 rules:
   - type: pipeline
     branches:
-      - main
+      - # branch is mandatory
 actions:
   - scan: dast
     site_profile: ''