public BooleanExpression isNull() {
        if (isnull == null) {
            isnull = Expressions.booleanOperation(Ops.IS_NULL, mixin);
        }
        return isnull;
    }
Create a {@code this is null} expression

@return this is null
querydsl-core/src/main/java/com/querydsl/core/types/dsl/SimpleExpression.java