backend: mark blocking=null, accessible=null web measurements as failed

[bassosimone]

Further details will follow. Very briefly, this issue is related to https://github.com/ooni/backend/issues/627.

This issue was actually wrong. We reverted ~immediately the change and we're working on a different solution. See below for more information on why the issue is wrong.

[bassosimone]

This issue was wrong. When blocking and accessible are None, the probe does not know. This is different from failed.

The pipeline should prioritize the fingerprints over the probe's own assessment of a measurement.

[FedericoCeratto]

Implemented in https://github.com/ooni/pipeline/pull/396 If test_keys -> blocking or test_keys -> accessible are null the msmt should be failed unless it's flagged confirmed by the fingerprinting