transcript: show sender annotation if interrupted by date annotation

src/transcript/transcript-drawing-area.vala

@@ -339,6 +339,7 @@ private class TranscriptDrawingArea : Widget
                 var date_item = new DateItemLayout(date.to_local().format("%b %d, %Y at %H:%M"), this, max_width);
                 items.add(date_item);
                 last_date = date;
+                last_sender = null;
             }
 
             // Sender Annotation