[flang] Fix context less NULL() lowering

The current context less lowering of NULL is producing invalid code
(can lead to reading outside of allocated memory): it is casting
a simple pointer to a descriptor address.
Later, reads are made to this descriptor. It used to be "OK" when
fir.load of fir.box were no-ops, but this was incorrect, and the
fir.load codegen is known doing a copy, and read the whole descriptor
data, not only the base address.

The previous patch that allowed fir.box<None> allocation, this
code fix this by allocating an actual fir.box<None>.

Note: this is still an overkill way to lower foo(null()). HLFIR
lowering always contextualize NULL() lowering leading to much simpler
code:
```
%absent = fir.absent fir.box<T>
fir.call @foo(%absent)
```

Differential Revision: https://reviews.llvm.org/D147239