[mlir][llvm] Replace NullOp by ZeroOp (#67183)

This revision replaces the LLVM dialect NullOp by the recently
introduced ZeroOp. The ZeroOp is more generic in the sense that it
represents zero values of any LLVM type rather than null pointers only.

This is a follow to https://github.com/llvm/llvm-project/pull/65508