update yaml.is_valid for changes in OPA 0.43.0

[srenatus]

It's now expected to be false instead of undefined.

When fixing this, we should be able to bump the version of OPA used in CI to 0.43.0 (or 0.44.0 when it's released).

[srenatus]

There are also new errors when running against OPA v0.60.0. See here while the logs are still there.