[llvm] [Analysis] Fix null ptr dereference when using WriteGraph without branch probability info (PR #104102)

[Arthur Eubanks via llvm-commits]

https://github.com/aeubanks approved this pull request.

I would have preferred an opt test, but looking at the places that construct `DOTFuncInfo`, they seem to always pass BFI, so lgtm

https://github.com/llvm/llvm-project/pull/104102