Prevent declaration of unused variables

[bendbennett]

Closes: #92

If the spec contains a single nested attribute that has no attributes defined within it, for example:

{
    "name": "details",
    "single_nested": {
        "computed_optional_required": "computed"
    }
}

The generated output, contains unused variable declarations. Specifically

func (t DetailsType) ValueFromObject(ctx context.Context, in basetypes.ObjectValue) (basetypes.ObjectValuable, diag.Diagnostics) {
	var diags diag.Diagnostics

	attributes := in.Attributes() // Unused
        ...

func (v DetailsValue) ToTerraformValue(ctx context.Context) (tftypes.Value, error) {
	attrTypes := make(map[string]tftypes.Type, 0)

	var val tftypes.Value  // Unused
	var err error          // Unused
        ...

This PR prevents the generation of variable declarations that are unused in the ValueFromObject(), and ToTerraformValue() methods for list, map, set, and single nested attributes, and list, set, and single nested blocks.

[austinvalle]

Should probably also add a changelog, but other than that looks great! 🚀

Can probably just release v0.3.1 after merging if you'd like 👍🏻

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