by default, an empty byte array (FT_BYTES) is represented as an empty string
thus, using "Apply as Filter / Selected" on such an item creates an
invalid display filter expression, e.g. dvb-ci.mmi.char_tbl==
represent an empty byte array as "" if we're compiling a display filter
expression
Bug: 11526
Change-Id: Ie94507a24a496e0c25bcdadfab72fdf9fb35958a
Reviewed-on: https://code.wireshark.org/review/10540
Reviewed-by: Martin Kaiser <wireshark@kaiser.cx>