Fix “waiting” series creation with non-null roles

In #a4bff75a314d04e85f7cd977076550a6dfa5373d I added
read- and write-roles to the series API, but forgot to
explicitly set them in the `create/announce` series
mutation. This lead to a deserialization error with
`null` values.

This fix adds the roles as empty arrays.

backend/src/api/model/series.rs

@@ -12,7 +12,7 @@ use crate::{
             acl::{self, Acl},
         },
     },
-    db::{types::{SeriesState as State}, util::impl_from_db},
+    db::{types::SeriesState as State, util::impl_from_db},
     model::{Key, ExtraMetadata},
     prelude::*,
 };
@@ -114,8 +114,8 @@ impl Series {
     pub(crate) async fn create(series: NewSeries, context: &Context) -> ApiResult<Self> {
         let selection = Self::select();
         let query = format!(
-            "insert into series (opencast_id, title, state, updated) \
-                values ($1, $2, 'waiting', '-infinity') \
+            "insert into series (opencast_id, title, state, updated, read_roles, write_roles) \
+                values ($1, $2, 'waiting', '-infinity', '{{}}', '{{}}') \
                 returning {selection}",
         );
         context.db(context.require_tobira_admin()?)