[llvm] [ADT] Fix an empty BitVector call getData assert `idx < size()' failed (PR #65505)

[Jakub Kuderski via llvm-commits]

https://github.com/kuhar commented:

+1 to @zero9178's request
> So I think just calling .data() and putting a comment that says something like "Make sure calling .data() is legal even on an empty BitVector" to make sure that we do not regress would be great.

We can also assert that that the arrayref is `.empty()`.
 

https://github.com/llvm/llvm-project/pull/65505