The json mode was returning the string `none` when unmerged and
an array for merged extensions. Return an empty array instead.
Instead of branching on the output format, always add an empty strv
for an unmerged hierarchy: the table code renders an empty strv as
the ersatz string ("-") in text mode and as an empty array in json
mode. This also changes the human-readable output from "none" to "-".
It also updated the TEST-50-DISSECT.sysext.sh test. Technically
this is a compatbility break but its also arguably a bugfix.
Michael Vogt
7efa326b7a