Implement toString() Method for Exists Operator to Ensure Consistent String Representation

[purquijo]

The Exists operator should override the toString() method in the same way that the Not operator does. Otherwise, the operator will not be correctly applied to the string representation.

[sisimomo]

I no longer have an active project using this library or Spring Filter. After reading the Spring Filter documentation, it seems that the "exists" operator no longer exists. @torshid, is that correct? If so, I’ll remove it from the library. Otherwise, it would be helpful to update the documentation so I can align this lib with the expected syntax.