networkctl: move strv_isempty() check into dump_list()

networkctl: move strv_isempty() check into dump_list()

Previously, each invocation of dump_list() was prefixed with a call to strv_isempty() to suppress invocation of the
function when the list is empty anyway. Move the check into the function itself, so that we can reduce the code a bit
in size.

(Also, prefix a couple of invocations we knowingly ignore return errors with a (void) cast).