Fix ControlMixin.isEmpty to check nested filters
Fix ControlMixin.isEmpty to check nested filters

Most controls manipulate a single filter, but a control may opt to
use a nested filter, such as with the free-text control. This change
ensures nested filters are properly assessed for emptiness.

Signed-off-by: Byron Ruth <e9d71f5ee7c92d6dc9e92ffdad17b8bd49418f98@devel.io>

chop-dbhi/cilantro,chop-dbhi/cilantro,chop-dbhi/cilantro