fix: allow null updates in discounts

[pKorsholm]

What

• allow nulls in the update dto for promotions for: ends_at, valid_duration and usage_limit

Why

• to enable the admin client to remove the previously mentioned properties using the medusa-react client

[adrien2p]

@pKorsholm could you update the types in the entity as well please 😊

[adrien2p]

@pKorsholm could we add an integration test for that please :)

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Latest commit: 17bf3ff7fcf4791791fcd3a9eb4e3c7cfe4328cf

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[olivermrbl]

Fixed in #1545 but we might as well include these tests :)