transform: flatten mangler: handle nil subfields

[dfinkel]

Fix the FlattenMangler so it doesn't populate intermediate struct-fields
of pointer-type. Since pointerification pretty much always converts
these fields to pointers, this is the common case.

There was a bit of missing accounting, so with two structs:

type A struct {
	Field1 *int
	Field2 *string
}
struct B struct{
	Field3 *A
}

B.Field3 was always populated with the zero-value, even if both
B.Field3_Field1 and B.Field3_Field2 are nil.

This change fixes the case where all underlying fields are nil, and
prevents those intermediate fields from getting set to non-nil values.