SearchDelegate: Remove it when pressing 'x'

Usually 'x' just clears the query, but if there is no query it is
strange that nothing happens on pressing it.

lib/widgets/note_search_delegate.dart

@@ -31,7 +31,11 @@ class NoteSearchDelegate extends SearchDelegate<Note> {
       IconButton(
         icon: Icon(Icons.close),
         onPressed: () {
+          if (query.isEmpty) {
+            close(context, null);
+          } else {
             query = '';
+          }
         },
       ),
     ];