This cop looks for delegations to predicate methods with `allow_nil`
option. When delegating with this option enabled, we ends up with true,
false and nil and this does not preserve the boolean nature of predicate
methods.

The preferred implementation is to define a method to handle `nil` and
returns the correct Boolean value instead.

delegate :is_foo?, to: :bar, allow_nil: true

def is_foo?
  return false unless bar
  bar.is_foo?
end

def is_foo?
  !!bar&.is_foo?
end