UserTask/Operations import doesn't finish if the last page contains only independent tasks

Given:

• there are independent user tasks present in the engine (not associated with a process instance)

When:

• the import runs

Then:

• the import executes empty elasticsearch bulks resulting in the error:

  15:22:32.055 [ElasticsearchImportJobExecutor-pool-0] ERROR o.c.o.s.e.j.i.RunningUserTaskElasticsearchImportJob - Error while executing import to elasticsearch
  org.elasticsearch.action.ActionRequestValidationException: Validation Failed: 1: no requests added;
  	at org.elasticsearch.action.ValidateActions.addValidationError(ValidateActions.java:26)
  	at org.elasticsearch.action.bulk.BulkRequest.validate(BulkRequest.java:551)
  	at org.elasticsearch.client.RestHighLevelClient.performRequest(RestHighLevelClient.java:1559)
  	at org.elasticsearch.client.RestHighLevelClient.performRequestAndParseEntity(RestHighLevelClient.java:1525)
  	at org.elasticsearch.client.RestHighLevelClient.bulk(RestHighLevelClient.java:416)
  	at org.camunda.optimize.service.es.writer.RunningUserTaskInstanceWriter.importUserTaskInstances(RunningUserTaskInstanceWriter.java:80)
  	at org.camunda.optimize.service.es.job.importing.RunningUserTaskElasticsearchImportJob.persistEntities(RunningUserTaskElasticsearchImportJob.java:25)
  	at org.camunda.optimize.service.es.job.ElasticsearchImportJob.executeImport(ElasticsearchImportJob.java:61)
  	at org.camunda.optimize.service.es.job.ElasticsearchImportJob.run(ElasticsearchImportJob.java:43)
  	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
  	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
  	at java.lang.Thread.run(Thread.java:748)
  

Expected:

• the import should finish while skipping these task instances and backoff normally until new data is available