Also updates go docs for the default tags and ignore tags environment variables constants

```console
% go test -v ./internal/provider/... -run=TestExpandIgnoreTags
?       github.com/hashicorp/terraform-provider-aws/internal/provider/fwprovider        [no test files]
=== RUN   TestExpandIgnoreTags
--- PASS: TestExpandIgnoreTags (0.00s)
PASS
ok      github.com/hashicorp/terraform-provider-aws/internal/provider   4.747s
```

These changes retain legacy behavior which returns nil when no keys or prefixes are set and, for non-nil return objects, returns nil for empty key or prefix lists rather than the zero value `KeyValueTags` struct. Also adds additional test cases to cover variations on empty keys/prefixes and duplicated values.