fix: improve console message when unlinking from directory without a …

[benhancock]

…netlify.toml

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Summary

Fixes #6895

Before, if a user called netlify unlink while linked to a site whose local directory did not have a netlify.toml, the site would unlink successfully but with a Unlinked undefined from <site-name> message. This is because when the local directory for a linked site does not have a netlify.toml,site.configPath is undefined in unlink.ts, and so undefined gets interpolated into the output message. We now check to see if site.siteConfig is truthy before interpolating the path to the netlify.toml, otherwise if it is not defined we output Unlinked from <site-name>.

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