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

Bug: 10651
Change-Id: I4d7e79d6fad909453e3c6fa4973e11a5fd3f7ca4
Reviewed-on: https://code.wireshark.org/review/5060
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>