PMM-14566 Skip empty fields in QAN response (PMM2 behaviour).

[JiriCtvrtka]

https://jira.percona.com/browse/PMM-14566

PMM PR: percona/pmm#5021

[JNKPercona]

Server docker: perconalab/pmm-server-fb:PR-4221-ee298ce
Client docker: perconalab/pmm-client-fb:PR-4221-ee298ce
Watchtower docker: perconalab/pmm-watchtower-fb:PR-4221-ee298ce
Client tarball: https://s3.us-east-2.amazonaws.com/pmm-build-cache/PR-BUILDS/pmm-client/pmm-client-PR-4221-ee298ce.tar.gz
Staging instance: https://pmm.cd.percona.com/job/pmm3-aws-staging-start/parambuild/?DOCKER_VERSION=perconalab/pmm-server-fb:PR-4221-ee298ce&CLIENT_VERSION=https://s3.us-east-2.amazonaws.com/pmm-build-cache/PR-BUILDS/pmm-client/pmm-client-PR-4221-ee298ce.tar.gz

[JNKPercona]

API tests have succeded: https://pmm.cd.percona.com/job/pmm3-api-tests/5194/

[JNKPercona]

Server docker: perconalab/pmm-server-fb:PR-4221-016afc9
Client docker: perconalab/pmm-client-fb:PR-4221-016afc9
Watchtower docker: perconalab/pmm-watchtower-fb:PR-4221-016afc9
Client tarball: https://s3.us-east-2.amazonaws.com/pmm-build-cache/PR-BUILDS/pmm-client/pmm-client-PR-4221-016afc9.tar.gz
Staging instance: https://pmm.cd.percona.com/job/pmm3-aws-staging-start/parambuild/?DOCKER_VERSION=perconalab/pmm-server-fb:PR-4221-016afc9&CLIENT_VERSION=https://s3.us-east-2.amazonaws.com/pmm-build-cache/PR-BUILDS/pmm-client/pmm-client-PR-4221-016afc9.tar.gz

[JNKPercona]

API tests have succeded: https://pmm.cd.percona.com/job/pmm3-api-tests/5203/