[db] get() should return null if not found

For a while, I was on the idea that returning something was better than return null (to prevent exceptions), however in practice is better to return null when an id doesn't exists.

Why not using Optional? Although I'm planning to add support to it, it could make the code more complex for the consumers. For example, instead of using List<User> it would become: List<Optional<User>>. For now I will stick to returning null for simplicity and add additional methods to allow the use of Optional.