[clang-tools-extra] [clang-doc] Avoid deref of invalid std::optional (PR #131939)

[Paul Kirth via cfe-commits]

ilovepi wrote:

### Merge activity

* **Mar 20, 5:02 PM EDT**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/131939).


https://github.com/llvm/llvm-project/pull/131939