The default value for a variable that can contain an object reference is null. But your question "What if an object is instantiated and it has no constrcutor" doesn't mean anything. You can only instantiate an object by calling one of its constructors, and it isn't possible for a class to not have any constructors.
And your other question is what does "null instanceof Object" return? I will leave you to write a one-line test program for that.
All classes have a constructor. If none are explicitly written into the class definition, then a default no-args constructor is provided by the compiler.
Class/instance variables for non-primitive types are automatically initialized to null. However, local variables (e.g., declared within a method) must be explicitly initialized before they are used or a compiler error will result.