fix: Requesting history being funky

As it turns out, some of the code set the prev_batch for rooms too
early to an empty string. For synapse this means "request from the start",
for conduit it is just an error. This commit fixes that by never resolving
null --> empty string, but instead throw an error.

lib/src/room.dart

@@ -372,7 +372,7 @@ class Room {
     this.membership = Membership.join,
     int notificationCount,
     int highlightCount,
-    String prev_batch,
+    this.prev_batch,
     this.client,
     this.notificationSettings,
     Map<String, BasicRoomEvent> roomAccountData,
@@ -383,7 +383,6 @@ class Room {
         _oldestSortOrder = oldestSortOrder,
         notificationCount = notificationCount ?? 0,
         highlightCount = highlightCount ?? 0,
-        prev_batch = prev_batch ?? '',
         roomAccountData = roomAccountData ?? <String, BasicRoomEvent>{},
         summary = summary ??
             RoomSummary.fromJson({
@@ -945,6 +944,9 @@ class Room {
   /// the historical events will be published in the onEvent stream.
   Future<void> requestHistory(
       {int historyCount = defaultHistoryCount, onHistoryReceived}) async {
+    if (prev_batch == null) {
+      throw 'Tried to request history without a prev_batch token';
+    }
     final resp = await client.getRoomEvents(
       id,
       prev_batch,
@@ -1105,7 +1107,6 @@ class Room {
       onInsert: onInsert,
     );
     if (client.database == null) {
-      prev_batch = '';
       await requestHistory(historyCount: 10);
     }
     return timeline;

lib/src/utils/room_update.dart

@@ -66,7 +66,7 @@ class RoomUpdate {
                   update.unreadNotifications?.notificationCount ?? 0,
               highlight_count: update.unreadNotifications?.highlightCount ?? 0,
               limitedTimeline: update.timeline?.limited ?? false,
-              prev_batch: update.timeline?.prevBatch ?? '',
+              prev_batch: update.timeline?.prevBatch,
               summary: update.summary,
             )
           : update is InvitedRoomUpdate
@@ -76,7 +76,7 @@ class RoomUpdate {
                   notification_count: 0,
                   highlight_count: 0,
                   limitedTimeline: false,
-                  prev_batch: '',
+                  prev_batch: null,
                   summary: null,
                 )
               : update is LeftRoomUpdate
@@ -86,7 +86,7 @@ class RoomUpdate {
                       notification_count: 0,
                       highlight_count: 0,
                       limitedTimeline: update.timeline?.limited ?? false,
-                      prev_batch: update.timeline?.prevBatch ?? '',
+                      prev_batch: update.timeline?.prevBatch,
                       summary: null,
                     )
                   : null;

test/client_test.dart

@@ -233,7 +233,7 @@ void main() {
 
       expect(roomUpdateList[1].id == '!696r7674:example.com', true);
       expect(roomUpdateList[1].membership == Membership.invite, true);
-      expect(roomUpdateList[1].prev_batch == '', true);
+      expect(roomUpdateList[1].prev_batch == null, true);
       expect(roomUpdateList[1].limitedTimeline == false, true);
       expect(roomUpdateList[1].notification_count == 0, true);
       expect(roomUpdateList[1].highlight_count == 0, true);