You can watch and vote on the Hibernate issue for strict validation. Schema validation so far is a tool to verify if the schema that you have is compatible to the schema that Hibernate ORM would produce. Obviously, a nullability constraint is not necessarily important as long as the data does not contain a null. Similarly, a missing nullable = false config on the Hibernate mapping side when you have a not-null constraint on your database shouldn’t be a big deal, because the error would be caught by the database ultimately.
If you want to diff your schema against Hibernate ORM expectations to understand if you need a migration, you should look into tools like Liquibase. Quarkus has great integrations with that tool that allow to generate change sets/logs quite easily.