Handle missing version

[therve]

If server_tokens is off in the configuration, the server header won't be
returned to us when querying the status endpoint. Instead of logging a
useless error, let's ignore it.

Fixes #5247

[codecov]

Codecov Report

Merging #5250 into master will decrease coverage by 3.48%.
The diff coverage is 100%.

Impacted Files	Coverage Δ
nginx/tests/test_unit.py	100% <100%> (ø)	⬆️
nginx/datadog_checks/nginx/nginx.py	89.61% <100%> (+2.73%)	⬆️
...ts/tooling/configuration/consumers/test_example.py
clickhouse/datadog_checks/clickhouse/utils.py
...checks_dev/tests/plugin/test_environment_runner.py
riakcs/datadog_checks/riakcs/riakcs.py
oracle/tests/test_oracle.py
...r/datadog_checks/openstack_controller/__about__.py
twistlock/datadog_checks/twistlock/twistlock.py
linkerd/datadog_checks/linkerd/linkerd.py
... and 847 more