Support nil pointers to structs in properties

Allow primary builders to reduce allocations of empty structures by
allowing nil pointers to concrete struct types.  Property readers will
not recurse into nil pointers, property writers will replace the nil
pointer with a pointer to the zero value of the pointer element type.

Allows a >50% primary builder time improvement with a trivial change in
Soong.

Change-Id: If6ad674bf7bf2a694c335378a074643a97d3c50b