When using the `atomically` method to combine multiple atomic operations into a single mongo operation, if zero atomic operations are created by the block, then mongoid issues an incorrect update operation with an empty document `{}`.
This completely erases the underlying document.

Expected Behaviour: There should be no update, and no loss of data.

Example:

class SomeModel
   include Mongoid::Document

  def perform_some_updates
     atomically do
        set(:foo, 'bar') if some_condition
        set(:baz: 'quux') if other_condition
        # etc
    end
end

If both of the conditionals in the example evaluate to false, then models data is unexpectedly lost.