Timeline: repair oldest/newest metrics if we fetch nothing

ts/state/selectors/items.ts

@@ -0,0 +1,20 @@
+// Copyright 2019-2020 Signal Messenger, LLC
+// SPDX-License-Identifier: AGPL-3.0-only
+
+import { createSelector } from 'reselect';
+
+import { StateType } from '../reducer';
+import { ItemsStateType } from '../ducks/items';
+
+export const getItems = (state: StateType): ItemsStateType => state.items;
+
+export const getUserAgent = createSelector(
+  getItems,
+  (state: ItemsStateType): string => state.userAgent as string
+);
+
+export const getPinnedConversationIds = createSelector(
+  getItems,
+  (state: ItemsStateType): Array<string> =>
+    (state.pinnedConversationIds || []) as Array<string>
+);