fix NPE in /messages by checking if all events were filtered out (#4330)

changelog.d/4330.bugfix

@@ -0,0 +1 @@
+fix NPE in /messages by checking if all events were filtered out

synapse/handlers/pagination.py

@@ -235,13 +235,7 @@ class PaginationHandler(object):
                 "room_key", next_key
             )
 
-        if not events:
-            defer.returnValue({
-                "chunk": [],
-                "start": pagin_config.from_token.to_string(),
-                "end": next_token.to_string(),
-            })
-
+        if events:
             if event_filter:
                 events = event_filter.filter(events)
 
@@ -252,6 +246,13 @@ class PaginationHandler(object):
                 is_peeking=(member_event_id is None),
             )
 
+        if not events:
+            defer.returnValue({
+                "chunk": [],
+                "start": pagin_config.from_token.to_string(),
+                "end": next_token.to_string(),
+            })
+
         state = None
         if event_filter and event_filter.lazy_load_members():
             # TODO: remove redundant members