client: Add default targets path
client: Add default targets path

There is no situation where an empty targets path makes sense, so it
should default to "targets".

Signed-off-by: Jonathan Rudenberg <3692bfa45759a67d83aedf0045f6cb635a966abf@titanous.com>

dmcgowan/gotuf,theupdateframework/go-tuf,runcom/gotuf,diogomonica/gotuf,cyli/gotuf,endophage/gotuf