print: do not try to add an empty hex stream to PDML output for a text only hf

Bug: 10651
Change-Id: If38af2879b9a20f4755a7843f20c7327d66c95b1
Reviewed-on: https://code.wireshark.org/review/5054
Reviewed-by: Evan Huus <eapache@gmail.com>
Tested-by: Pascal Quantin <pascal.quantin@gmail.com>
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>