[Maps] custom category ramp auto complete is broken

[nreese]

Looks like this is a regression caused by #59585. getAutocompleteService is returning null in the code snippet below

https://github.com/elastic/kibana/blob/master/x-pack/legacy/plugins/maps/public/layers/sources/es_source.js#L335

getValueSuggestions = async (field, query) => {
    try {
      const indexPattern = await this.getIndexPattern();
      return await getAutocompleteService().getValueSuggestions({
        indexPattern,
        field: indexPattern.fields.getByName(field.getRootName()),
        query,
      });
    } catch (error) {
      console.warn(
        `Unable to fetch suggestions for field: ${field.getRootName()}, query: ${query}, error: ${
          error.message
        }`
      );
      return [];
    }
  };

[elasticmachine]

Pinging @elastic/kibana-gis (Team:Geo)