[flang][hlfir] Fixed NULL() handling in structure constructor.

When an initializer value is missing for an allocatable component
in a structure constructor, the RHS is NULL() expression.
We should just skip this part of the initializer, since the component
must become unallocated (as it is from the initialization).
Runtime detected rank mismatch when we tried to pass NULL() box
RHS for assigning it to the unallocated component of rank 1, 2, etc.

Reviewed By: tblah

Differential Revision: https://reviews.llvm.org/D154906