[clang] [clang-format] Fix an assertion failure on comment-only config files (PR #163111)

[via cfe-commits]

llvmbot wrote:

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Failed to cherry-pick: 059f2df74898ff7f394dc8e60f746323499ae32b

https://github.com/llvm/llvm-project/actions/runs/18582710285

Please manually backport the fix and push it to your github fork.  Once this is done, please create a [pull request](https://github.com/llvm/llvm-project/compare)

https://github.com/llvm/llvm-project/pull/163111