PullingAsyncPipelineExecutor::pull do not return empty blocks

[vdimir]

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[robot-clickhouse-ci-2]

This is an automated comment for commit 36e193a with description of existing statuses. It's updated for the latest CI running

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Successful checks

Check name	Description	Status
Mergeable Check	Checks if all other necessary checks are successful	✅ success
Push to Dockerhub	The check for building and pushing the CI related docker images to docker hub	✅ success
Style Check	Runs a set of checks to keep the code style clean. If some of tests failed, see the related log from the report	✅ success

Check name	Description	Status
CI running	A meta-check that indicates the running CI. Normally, it's in success or pending state. The failed status indicates some problems with the PR	⏳ pending
Fast test	Normally this is the first check that is ran for a PR. It builds ClickHouse and runs most of stateless functional tests, omitting some. If it fails, further checks are not started until it is fixed. Look at the report to see which tests fail, then reproduce the failure locally as described here	❌ failure