Delete with executeSingle function may cause NullPointerException

[wangzhengyi]

If user use
new Delete().from(StudentDAO.class).where("age = 100").executeSingle()

and the student is not exist, this may cause NullPointerException.

I create this PR for this problem. link:#510

[uncledoc]

It is wrong way. executeSingle() - add to the delete statement LIMINT 1 and you will have syntax error