fix timeline search with empty text box should do nothing (#8262)

* fix timeline search with empty text box should do nothing

* test SearchBar component

* fix lint error

* Update SearchBar-test.tsx

Co-authored-by: Travis Ralston <travisr@matrix.org>

src/components/views/rooms/SearchBar.tsx

@@ -78,6 +78,7 @@ export default class SearchBar extends React.Component<IProps, IState> {
     }
 
     private onSearch = (): void => {
+        if (!this.searchTerm.current.value.trim()) return;
         this.props.onSearch(this.searchTerm.current.value, this.state.scope);
     };