feat(sdk): LatestEventValue::None overwrites previous value.

This patch removes an optimisation that was considering
`LatestEventValue::None` as a value that should no overwrite the
previous value if any. This is a mistake: if the event cache is cleared,
it won't be possible to calculate a new value, and the previous value
must be overwritten.

Author: Hywan

crates/matrix-sdk/src/latest_events/latest_event.rs

@@ -81,12 +81,7 @@ impl LatestEvent {
         )
         .await;
 
-        if let LatestEventValue::None = new_value {
-            // The new value is `None`. It means no new value has been
-            // computed. Let's keep the old value.
-        } else {
-            self.value.set(new_value).await;
-        }
+        self.value.set(new_value).await;
     }
 }