message: hide RoomSubList if has 0 tiles and either searchFilter or emptyContent
diff: /*
  -Copyright 2017 Vector Creations Ltd
  Copyright 2015, 2016 OpenMarket Ltd
  +Copyright 2017 Vector Creations Ltd
  +Copyright 2018 New Vector Ltd
  Licensed under the Apache License, Version 2.0 (the "License");
  you may not use this file except in compliance with the License.
  @@ -349,8 +350,14 @@ var RoomSubList = React.createClass({
  var label = this.props.collapsed ? null : this.props.label;
  let content;
  - if (this.state.sortedList.length === 0 && !this.props.searchFilter && this.props.extraTiles.length === 0) {
  + if (this.state.sortedList.length === 0 && this.props.extraTiles.length === 0) {
  + // if no search filter is applied and there is a placeholder defined then show it, otherwise show nothing
  + if (!this.props.searchFilter && this.props.emptyContent) {
  content = this.props.emptyContent;
  + } else {
  + // don't show an empty sublist
  + return null;
  + }
  } else {
  content = this.makeRoomTiles();
  content.push(...this.props.extraTiles);
  