How can I create an Arel predicate representing the empty string?

In active record I can do the following:

User.where("").to_sql
=> "SELECT \"users\".* FROM \"users\" " 

I'd like to make a method that uses Arel to create a predicate to pass as an argument to where. In some cases, I'd like that method to return a predicate with the property,

pred.to_sql
=> ""

that is, I would like the empty predicate. My goal is to be able to have the following,

User.where(my_method).to_sql

always be a valid query.

Using Arel, how can I create this empty predicate? I had a hard time googling this, because empty tends to fill the search with questions about empty query results.