test: add missing prop schema

core/modules/system/tests/modules/sdc_test/components/just-a-slot/just-a-slot.component.yml

 $schema: https://git.drupalcode.org/project/drupal/-/raw/HEAD/core/assets/schemas/v1/metadata.schema.json
 name: Just a slot
+props:
+  type: object
+  properties: {}
 slots:
   the_slot: {}