`MPI.REQUEST_NULL` only pretends to be a constant

```
using MPI
MPI.Init()

reqs = Vector{MPI.Request}(undef, 2)
fill!(reqs, MPI.REQUEST_NULL)

# first foot-gun `reqs[1]` and `reqs[2]` are now aliases

MPI.Irecv!(Ref(1.0), MPI.COMM_WORLD, reqs[1])
```

```
julia> reqs
2-element Vector{MPI.Request}:
incomplete request
incomplete request
```

But worse...

```
julia> MPI.REQUEST_NULL
MPI.Request: incomplete request
```