skip 'not' with 'nil opts'

`Model.not ` should also skip(similar to `where`) `not` scope from the `WhereChain`, if it receives `nil`, like:

    Before:
      User.where.not(nil) # => 'SELECT `users`.* FROM `users`  WHERE (NOT (NULL))'

    After:
      User.where.not(nil) # => 'SELECT `users`.* FROM `users`'