Prevent RoomDirectory from exploding when filterString is wrongly nullified

src/components/structures/RoomDirectory.tsx

@@ -370,7 +370,7 @@ export default class RoomDirectory extends React.Component<IProps, IState> {
 
     private onFilterChange = (alias: string) => {
         this.setState({
-            filterString: alias || null,
+            filterString: alias || "",
         });
 
         // don't send the request for a little bit,
@@ -389,7 +389,7 @@ export default class RoomDirectory extends React.Component<IProps, IState> {
     private onFilterClear = () => {
         // update immediately
         this.setState({
-            filterString: null,
+            filterString: "",
         }, this.refreshRoomList);
 
         if (this.filterTimeout) {