Update filtering helper functions definition to be more direct

[sabit990928]

What and Why

I changed the phrasing in the wine-cellar exercise for helper functions to clarify the requirements. Because while solving the problem it seemed natural to extend filtering with additional match on nil case. Which seems like not intentional as my mentor mentioned.

Maybe my wording is not best, but I think this part can be improved, so any feedback welcomed.

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Automated comment created by PR Commenter 🤖.

[IsaacG]

+cc @exercism/elixir

[jiegillet]

Thanks for the contribution, "should not" is indeed clearer than "do not need". I'm not sure the second sentence is needed.

[jiegillet]

Thank you :)