use the 'pf -ef' compatible default when the descriptor argument
of Top() is []string{""} or []string{}
why:
the call to Top() in
pkg/api/handlers/compat/containers_top.go#L62C3-L62C3
passes []string{""} descriptors whenever ps_args is empty (the
default value for libpod requests) because of golang strings.Split()
semantics.
[NO NEW TESTS NEEDED]
Signed-off-by: Stefan Grundmann <sg2342@googlemail.com>