don't print the interface description if it's null

(named pipes added with -i <pipe name> don't have such a description)

Change-Id: I5986c607f5103b800c4353fd3cf01f24149e2e5c
Reviewed-on: https://code.wireshark.org/review/2940
Reviewed-by: Martin Kaiser <wireshark@kaiser.cx>
Tested-by: Martin Kaiser <wireshark@kaiser.cx>