CodeownersLinter should not require an owner

It's completed valid syntax - even according to GitHub's built-in `CODEOWNERS` linter - to have a path reference without an owner e.g.,

```codeowners
/Cargo.toml @heaths
/Cargo.lock
```

But our linter requires it, forcing us to add a baseline file when it seems it shouldn't be an error anyway: only people with write permissions can merge anyway, and only their PR approvals count toward satisfying branch policies.

See https://github.com/Azure/azure-sdk-for-rust/pull/2774 for more context.

Start with CodeownersLinter and trace how it validates a path-only CODEOWNERS entry. Compare the expected behavior with GitHub's built-in CODEOWNERS linter and the linked pull request #2774 for context. Done means an entry such as /Cargo.lock is accepted without requiring an owner.

Written by the indexing model from the issue text.