[clang][nullability] Don't return null fields from `getReferencedDecls()`. (#94983)

The patch includes a repro for a case where we were returning a null
`FieldDecl`
when calling `getReferencedDecls()` on the `InitListExpr` for a union.

Also, I noticed while working on this that `RecordInitListHelper` has a
bug
where it doesn't work correctly for empty unions. This patch also
includes a
repro and fix for this bug.