fix(lsp): error when opening a rust file that does not exist

[mrcjkb]

Fixes #404.

[github-actions[bot]]

Review Checklist

Does this PR follow the Contribution Guidelines? Following is a partial checklist:

Proper conventional commit scoping:

• For example, fix(lsp): some lsp-related bugfix

• [ ] Pull request title has the appropriate conventional commit prefix.

If applicable:

• [ ] Tested
  • [ ] Tests have been added.
  • [ ] Tested manually (Steps to reproduce in PR description).
• [ ] Updated documentation.
• [ ] Updated CHANGELOG.md