plugin fails to start with default hosts setting

[kares]

When hosts isn't specified (localhost:9200 is expected to be used) plugin fails to start properly:
(Elasticsearch::Transport::Transport::Error unable to retrieve an initial connection from the pool)

[2021-08-12T13:29:56,788][INFO ][logstash.javapipeline    ][main] Pipeline started {"pipeline.id"=>"main"}
[2021-08-12T13:29:56,869][INFO ][logstash.agent           ] Pipelines running {:count=>1, :running_pipelines=>[:main], :non_running_pipelines=>[]}
[2021-08-12T13:29:56,947][ERROR][logstash.javapipeline    ][main][4e805afb4960a70553434dc5d7ffc52f8b2410239d6cfc4372f5a66ea701b94c] A plugin had an unrecoverable error. Will restart this plugin.
  Pipeline_id:main
  Plugin: <LogStash::Inputs::Elasticsearch index=>"kibana_sample_data_logs", password=><password>, id=>"4e805afb4960a70553434dc5d7ffc52f8b2410239d6cfc4372f5a66ea701b94c", size=>2, user=>"elastic", enable_metric=>true, codec=><LogStash::Codecs::JSON id=>"json_2ddd7d2d-4ec1-44fb-a9d8-359217280234", enable_metric=>true, charset=>"UTF-8">, query=>"{ \"sort\": [ \"_doc\" ] }", scroll=>"1m", docinfo=>false, docinfo_target=>"@metadata", docinfo_fields=>["_index", "_type", "_id"], connect_timeout_seconds=>10, request_timeout_seconds=>60, socket_timeout_seconds=>60, ssl=>false>
  Error: Cannot get new connection from pool.
  Exception: Elasticsearch::Transport::Transport::Error
  Stack: /home/kares/workspace/work/elastic/logstash-7.12.1/vendor/bundle/jruby/2.5.0/gems/elasticsearch-transport-5.0.5/lib/elasticsearch/transport/transport/base.rb:254:in `perform_request'
/home/kares/workspace/work/elastic/logstash-7.12.1/vendor/bundle/jruby/2.5.0/gems/elasticsearch-transport-5.0.5/lib/elasticsearch/transport/transport/http/manticore.rb:67:in `perform_request'
/home/kares/workspace/work/elastic/logstash-7.12.1/vendor/bundle/jruby/2.5.0/gems/elasticsearch-transport-5.0.5/lib/elasticsearch/transport/client.rb:131:in `perform_request'
/home/kares/workspace/work/elastic/logstash-7.12.1/vendor/bundle/jruby/2.5.0/gems/elasticsearch-api-5.0.5/lib/elasticsearch/api/actions/search.rb:183:in `search'
/home/kares/workspace/work/elastic/logstash-7.12.1/vendor/bundle/jruby/2.5.0/gems/logstash-input-elasticsearch-4.9.1/lib/logstash/inputs/elasticsearch.rb:349:in `search_request'
/home/kares/workspace/work/elastic/logstash-7.12.1/vendor/bundle/jruby/2.5.0/gems/logstash-input-elasticsearch-4.9.1/lib/logstash/inputs/elasticsearch.rb:272:in `do_run_slice'
/home/kares/workspace/work/elastic/logstash-7.12.1/vendor/bundle/jruby/2.5.0/gems/logstash-input-elasticsearch-4.9.1/lib/logstash/inputs/elasticsearch.rb:250:in `do_run'
/home/kares/workspace/work/elastic/logstash-7.12.1/vendor/bundle/jruby/2.5.0/gems/logstash-input-elasticsearch-4.9.1/lib/logstash/inputs/elasticsearch.rb:238:in `run'
/home/kares/workspace/work/elastic/logstash-7.12.1/logstash-core/lib/logstash/java_pipeline.rb:405:in `inputworker'
/home/kares/workspace/work/elastic/logstash-7.12.1/logstash-core/lib/logstash/java_pipeline.rb:396:in `block in start_input'

Sample reproducer (tested on LS 7.14.0 as well as older):
input { elasticsearch { index => 'kibana_sample_data_logs' size => 2 user => 'elastic' password => '...' } }

NOTE: adding hosts => 'localhost:9200' explicitly resolves the error.

Issue seems to relate to #133 (fixed a while back) - pool simply returns no connection.