Insert Data From Keyboard And Check Whether Any Of Constructor Fields Remained Empty

I'm trying to use in the constructor inserts from keyboard and then check whether any of the constructors fields remained empty. Let say my constructor is:

public SomeClass(String a, String b, String c)throws SomeClassException{
if (a == null || b == null || c == null) {throw new SomeClassException("Some message"); // here is the empty test
} //...

Thus I prepared method: insertConstruct() as follows:

public static String insertConstruct(){
String lettering = null;
Scanner sc = new Scanner(System.in);
lettering= sc.nextLine();
return lettering;
}

And during creating the new object in some main method I call:

// ...
ClassName obj1;
obj1 = new ClassName(insertConstruct(),insertConstruct(),insertConstruct());
//...

The problem is not with inserting data but with leaving the field empty. When I pass any field just with keyboards Enter, the empty test doesnt return me any message about it. While it works if I build object with the constructor like:

// ...
className obj1;
obj1 = new ClassName("word a",null,"word c");
//...

What might case this failure?