By default the compiler generates the default constructor for you, so if you don't want to specify any special actions( initializing of members is not the point here ), then you have not to specify the constructor.
Other thing is that some classes should have a consistent state. For example you have a Book class. There is no sence to create a book with no title, so it is necessary to specify a constructor with string parameter:
public Book(String name) {
this.name = name;
}
As for default constructors they may be necessary if you should serialize your class or use it in marshalling/unmarshalling(JAXB requires an empty default constructor).
If that's not the point and your class hasn't what is called a consistent state, so empty default constructor is absolutely unnecessary to declare.
You should remember that default constructor is public by default, so consider decalring an explicit one if you want some restrictions to this.
Also if your class is rather long you can consider declaring an empty default constructor to increase readability.