Fix timeline & notifs panel spuriously being empty

Only claim there's nothing to display once we've failed to back
paginate, otherwise we'll show the empty message instead of the
MessagePanel and therefore never try to back-paginate.

src/components/structures/TimelinePanel.js

@@ -993,7 +993,7 @@ var TimelinePanel = React.createClass({
             );
         }
 
-        if (this.state.events.length == 0) {
+        if (this.state.events.length == 0 && !this.state.canBackPaginate && this.props.empty) {
             return (
                     <div className={ this.props.className + " mx_RoomView_messageListWrapper" }>
                         <div className="mx_RoomView_empty">{ this.props.empty }</div>