task(event cache): ignore add_initial_events() when the event cache storage has been enabled

Not worth testing IMO, since this is about the "temporary" API we're
going to remove in subsequent patches.

Author: bnjbvr

crates/matrix-sdk/src/event_cache/mod.rs

@@ -312,6 +312,11 @@ impl EventCache {
         events: Vec<SyncTimelineEvent>,
         prev_batch: Option<String>,
     ) -> Result<()> {
+        // If the event cache's storage has been enabled, do nothing.
+        if self.inner.store.get().is_some() {
+            return Ok(());
+        }
+
         let room_cache = self.inner.for_room(room_id).await?;
 
         // We could have received events during a previous sync; remove them all, since