code: public Criteria exists(boolean shouldExist) {
          this.criteriaType = RelationalOperator.EXISTS;
          this.right = ValueNode.toValueNode(shouldExist);
          this.left = left.asPathNode().asExistsCheck(shouldExist);
          return this;
      }
docs: Check for existence (or lack thereof) of a field.
  
  @param shouldExist
  @return the criteria