indicate that a plugin's form is intentionally empty

Problem/Motivation

If a suggester plugin has no configuration form elements, then the details element for that plugin's configuration in the form looks confusing.

Steps to reproduce

Try the search_api_solr_spellcheck plugin from the search_api_solr module.

Proposed resolution

Add a markup element in SearchEditForm if the suggester plugin returned an empty form, saying something like

> This suggester plugin has no configuration.