[LLDB] Fix crash when using tab completion on class variables (#83234)

We weren't checking to see if the partial_path was empty before adding
completions and this led to crashes when the class object and a variable
both start with the same substring.

Fixes [#81536](https://github.com/llvm/llvm-project/issues/81536

)

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Co-authored-by: Michael Buch <michaelbuch12@gmail.com>