For a credentials helper plugin to be useful with Terraform 0.13+, we
need to cope with the case of having no credentials for a host without
this being an error. This is to allow the public Terraform Registry to
be accessed without supplying a token.
The way to implement this is to respond to queries for credentials for a
host which has no credentials stored with an empty object and a success
exit code. This contradicts the previous documentation, which calls for
an error response in this case.
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