[MOB-769] Fixed sku not being nullable

[carvalho2707]

What does this PR do?

This PR fixes a bug introduced in the release where sku was considered to be non null and it shouldn't

Database changed?

No

Where should the reviewer start?

• [ ] AppcoinsRewardsBuyPresenter.kt

How should this be manually tested?

Flow on how to test this or QA Tickets related to this use-case: MOB-769

What are the relevant tickets?

Tickets related to this pull-request: MOB-769

Questions:

Does this add new dependencies which need to be added? (Eg. new libs, new keys, etc.)

Code Review Checklist

• [ ] Architecture
• [ ] Documentation on public interfaces
• [ ] Database changed?
• [ ] If yes - Database migration?
• [ ] Remove comments & unused code & forgotten testing Logs
• [ ] Codestyle
• [ ] Functional QA tests pass