refactor(sdk): Move reached_start closer to where it is used.

This patch moves `reached_start` closer to where it is used: it is true
if and only if the chunk content is `Items`, it is false for a `Gap`.

Author: Hywan

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

@@ -920,10 +920,6 @@ mod private {
 
             let chunk_content = new_first_chunk.content.clone();
 
-            // We've reached the start on disk, if and only if, there was no chunk prior to
-            // the one we just loaded.
-            let reached_start = new_first_chunk.previous.is_none();
-
             if let Err(err) = self.events.insert_new_chunk_as_first(new_first_chunk) {
                 error!("error when inserting the previous chunk into its linked chunk: {err}");
 
@@ -949,7 +945,9 @@ mod private {
                 ChunkContent::Items(events) => LoadMoreEventsBackwardsOutcome::Events {
                     events,
                     timeline_event_diffs,
-                    reached_start,
+                    // We've reached the start on disk, if and only if, there was no chunk prior to
+                    // the one we just loaded.
+                    reached_start: self.events.chunks().next().unwrap().lazy_previous().is_none(),
                 },
             })
         }