Server::getAutocompleteSuggestions adds fields to query even when no fields specified

Problem/Motivation

The fields property of the $query is updated every time there is a value in $this->configuration['fields']. And there always is, even when no boxes have been checked in the configuration form:

   fields:
     rendered_item: '0'
     tag_names: '0'
     title_1: '0'

This is a problem later on when determining full text search fields for autocomplete suggestion queries, as \Drupal\search_api\Backend\BackendPluginBase::getQueryFulltextFields() performs an array_intersect() on $query->getFullTextFields() against $query->getIndex()->getFulltextFields() if the former returns values. If the query does not return values, the index' full text fields are returned, which is the purpose of leaving it empty, as illustrated by the config field description (emphasis mine):

Select the fields which should be searched for matches when looking for autocompletion suggestions. Leave blank to use the same fields as the underlying search.

Proposed resolution

Add array_filter() calls in appropriate places to make sure the 'empty' values don't end up as fields in the $query object.

Remaining tasks

Review patch provided in my next comment.

User interface changes

None

API changes

None

Data model changes

None