Allow returning nil from a named_scope lambda

Returning an empty hash from a named_scope lambda basically means "do nothing."

I have found this to be a good pattern for disabling a named_scope under certain conditions, most commonly when the argument to the named_scope lambda was nil.

I propose returning nil should have the same effect as returning an empty hash. They both should mean "do nothing."

# with current implementation
named_scope :with_town_like, lambda { |term|
  if !term.blank?
    { :conditions => ['town LIKE ?', term] }
  else
    {}
  end
}


# new style, with nil as acceptable return value
named_scope :with_town_like, lambda { |term|
  { :conditions => ['town LIKE ?', term] } unless term.blank?
}