Add some tests and remove assert.

[TheMeinerLP]

This improves the handling of Namespace Keys in Minestom. Assert is misbehavior in that case and should never in production code. In 99% of cases, the assert will never match because of the default Java behaviour.
Check out following tutorial to understand why assert is not triggering: https://www.baeldung.com/java-assert

[sonarcloud]

Quality Gate failed

Failed conditions
C Maintainability Rating on New Code (required ≥ A)

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