[RDA] Try to fix build (NFC)

fuchsia-x86_64-linux builder fails with:

/b/fuchsia-x86_64-linux/llvm.src/llvm/include/llvm/ADT/TinyPtrVector.h:85:15:
error: no matching conversion for C-style cast from 'nullptr_t' to 'llvm::ReachingDef'
    RHS.Val = (EltTy)nullptr;

Let's see whether adding an explicit nullptr_t constructor helps.