client-api: Don't skip all no-event timelines serializing sync response

There are some cases where a timeline can have zero events but should
still be included in the api response. For example, when calling `/sync`
with a filter that rejects all events after `since`, but does not
necessarily reject all events in the room's history, the response should
include a `prev_batch` field so that the client can search for earlier
events matching the filter using `/messages`.

crates/ruma-client-api/CHANGELOG.md

@@ -4,6 +4,8 @@ Bug fixes:
 
 - Don't require the `failures` field in the
   `ruma_client_api::keys::upload_signatures::Response` type.
+- `sync::sync_events::v3::Timeline::is_empty` now returns `false` when the
+  `limited` or `prev_batch` fields are set.
 
 Breaking changes:
 

crates/ruma-client-api/src/sync/sync_events/v3.rs

@@ -360,8 +360,11 @@ impl Timeline {
     }
 
     /// Returns true if there are no timeline updates.
+    ///
+    /// A `Timeline` is considered non-empty if it has at least one event, a
+    /// `prev_batch` value, or `limited` is `true`.
     pub fn is_empty(&self) -> bool {
-        self.events.is_empty()
+        !self.limited && self.prev_batch.is_none() && self.events.is_empty()
     }
 }