Null Value accepting in String Type

Hi All,
I have created a Query Params and one of the Params is mentioned as below,
Testing:
description:
required: true
type: string
example: “ABCDEFG”
During the testing, I could see null value was getting accepted when i pass for this field. Could you please confirm whether null is a acceptable one when the Type is String ?? If it is a acceptable will there any way i can block Null for this field where it accepts only characters whereas null and empty quotes are not accepted ?? Thanks.

Note:- I tried with all these values Null, null and nil . All values are accepted and it doesnt throw error.