Prevent lockfile creation when project folder is missing

[cknitt]

Bail out of lock::get when the requested project root does not exist so ReScript no longer creates stray folders during failed builds.

Fixes #7926

[cknitt]

@codex review

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commit: e0161f9

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. You're on a roll.

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[nojaf]

Looks good, don't forget the changelog.