Ignore empty label_to_hostname label value

[ahmed-mez]

What does this PR do?

• Do not add a host tag when hostname is None and the label_to_hostname label's value is ""

Motivation

• Avoid having incomplete and incorrect host tags with label_to_hostname_suffix only (host:-<cluster-name> in case of KSM)
• When hostname is None and the label_to_hostname label's value is "" we shouldn't add a host tag

Additional Notes

Could be QAed using KSM and the metric kube_pod_status_unschedulable

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• PR title must be written as a CHANGELOG entry (see why)
• Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• PR must have changelog/ and integration/ labels attached

[codecov]

Codecov Report

Merging #7232 into master will increase coverage by 4.22%.
The diff coverage is n/a.

Impacted Files	Coverage Δ
disk/tests/common.py	64.28% <0.00%> (-35.72%)	⬇️
disk/tests/conftest.py	85.71% <0.00%> (-11.43%)	⬇️
disk/datadog_checks/disk/disk.py	83.66% <0.00%> (-3.59%)	⬇️
disk/tests/test_filter.py	96.45% <0.00%> (-3.55%)	⬇️
dns_check/datadog_checks/dns_check/dns_check.py	86.45% <0.00%> (-2.09%)	⬇️
twemproxy/tests/test_twemproxy.py
ibm_db2/tests/test_integration.py
rethinkdb/datadog_checks/rethinkdb/types.py
http_check/datadog_checks/http_check/adapters.py
ssh_check/datadog_checks/ssh_check/__about__.py
... and 713 more

[therc]

Sorry to post on a PR that's almost one year old, but this is where there's the most context. What are the expectations if an agent sends a metric without any host tags at all? I'm asking because I found some odd kubernetes_state metrics from the first attempt to run a datadog-agent on a brand new node (it failed the health check within a few minutes and it worked fine after a restart). In those metrics, the host was set to an EC2 instance ID that I can't find anywhere in EKS, Datadog or ASG logs. No other metric or integration seems to have used it (metrics summary is not too helful here, maybe because there seems to be data for just those four minutes). I was wondering if perhaps this might have been set on the server side, hence my question.