pdl: Fix generation of packets with children AND no payload

The parser and serialize both have issues in this case
with parts of the generated code assuming the presence of the
payload and other not.

The added test packet_decl_parent_with_no_payload
validates the generated code for this condition.

Bug: 279494407
Test: atest pdl_generated_files_compile
Change-Id: Ibbd97441a51fc499250e2072ec8da7f3b7806970