archive: Fix typing for prev_sender variable.

We know that via the `AbstractMessage` class that `sender`
is of the type `UserProfile`.  We type this as `Optional`
to tell mypy that the operands to the right of the first
`or` can indeed be evaluated within the following `for` loop.

zerver/views/archive.py

@@ -1,11 +1,11 @@
-from typing import List
+from typing import List, Optional
 
 from django.http import HttpRequest, HttpResponse
 from django.shortcuts import render
 from django.template import loader
 from zerver.lib.streams import get_stream_by_id
 
-from zerver.models import Message, get_stream_recipient
+from zerver.models import Message, get_stream_recipient, UserProfile
 from zerver.lib.avatar import get_gravatar_url
 from zerver.lib.response import json_success
 from zerver.lib.timestamp import datetime_to_timestamp
@@ -52,7 +52,7 @@ def archive(request: HttpRequest,
         return get_response([], True, stream.name)
 
     rendered_message_list = []
-    prev_sender = None
+    prev_sender = None  # type: Optional[UserProfile]
     for msg in all_messages:
         include_sender = False
         status_message = Message.is_status_message(msg.content, msg.rendered_content)