Don't cache stuff we didn't find

src/db/roomstore.ts

@@ -175,7 +175,9 @@ export class DbRoomStore {
                 remote: remoteId ? new RemoteStoreRoom(remoteId, row as any) : null,
             });
         }
+        if (res.length > 0) {
             this.entriesMatrixIdCache.set(matrixId, {e: res, ts: Date.now()});
+        }
         return res;
     }