Refactor NullRelation#calculate

[kamipo]

Before:

    def calculate(operation, _column_name)
      if [:count, :sum].include? operation
        group_values.any? ? Hash.new : 0
      elsif [:average, :minimum, :maximum].include?(operation) && group_values.any?
        Hash.new
      else
        nil
      end
    end

After:

    def calculate(operation, _column_name)
      case operation
      when :count, :sum
        group_values.any? ? Hash.new : 0
      when :average, :minimum, :maximum
        group_values.any? ? Hash.new : nil
      end
    end

[rails-bot]

r? @eileencodes

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