How can I make a nil compatible completion handler?

I created a function with a completion handler. Sometimes I need to use the completion block, sometimes not.

Here is the function:

func numberCheck(number: String , completion : @escaping (Bool)->()){
     //some task do here 
    completion(true)
}

use:

numberCheck(number: "77" , completion: {_ in
    //some task do here 
})

But I want to leave it completion block nil:

numberCheck(number: "77" ,  completion: nil)

but it is gives me an error:

Nil is not compatible with expected argument type '(Bool) -> ()'