Test all backends with an empty store_id

Most VSS users don't actually care about the `store_id` - they have
some data which they want to store for themselves (keyed on the
authenticated user id) and that's it. There's not really any reason
to force them to specify a `store_id`, the empty string is just as
valid as any other.

In the previous commit we allowed empty `store_id`s in the postgres
backend, here we add tests (randomly) with empty `store_id`s in the
standardized backend tests.

Author: TheBlueMatt

rust/api/src/kv_store_tests.rs

@@ -552,8 +552,12 @@ pub struct TestContext<'a> {
 impl<'a> TestContext<'a> {
 	/// Creates a new [`TestContext`] with the given [`KvStore`] implementation.
 	pub fn new(kv_store: &'a dyn KvStore) -> Self {
-		let store_id: String = (0..7).map(|_| thread_rng().sample(Alphanumeric) as char).collect();
-		TestContext { kv_store, user_token: "userToken".to_string(), store_id }
+		let store_id_len = thread_rng().gen_range(0..6);
+		let store_id: String =
+			(0..store_id_len).map(|_| thread_rng().sample(Alphanumeric) as char).collect();
+		let user_token: String =
+			(0..7).map(|_| thread_rng().sample(Alphanumeric) as char).collect();
+		TestContext { kv_store, user_token, store_id }
 	}
 
 	async fn get_object(&self, key: &str) -> Result<KeyValue, VssError> {