When `client-id` (or the deprecated `app-id`) resolves to an empty
string, for example because a secret or variable is not available in the
workflow context, the error message from `@octokit/auth-app` is not very
helpful:
```
[@octokit/auth-app] appId option is required
```
A validation check was added recently to catch this earlier, but its
message could be more informative:
```
Either 'client-id' or 'app-id' input must be set
```
This updates the message to clarify that the value resolved to empty and
nudges users toward checking their secret or variable availability:
```
The 'client-id' input must be set to a non-empty string. If using a secret or variable, ensure it is available in this workflow context.
```
Closes#249
---------
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
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