Search fail if key `filters:` is not present

Hi,

When using the admin plugin, when I remove the only tag in the `Category filter`, the line `filters:` is deleted from `simplesearch.yaml`.

When this line is not present, the search fail even when there are result.

After adding the line `filters:` manually, the search works again.

And when editing another field in the admin plugin, the line is removed again.