Fix: optional descriptions in axiom provider Resources

[a-khaledf]

Currently When you create any resource without description, it will fail with the following error

        Error: Provider produced inconsistent result after apply

        When applying changes to axiom_dataset.test_without_description, provider
        "provider[\"registry.terraform.io/hashicorp/axiom\"]" produced an unexpected
        new value: .description: was null, but now cty.StringVal("").

This PR is the fix for that