The no-args constructor for UnsupportedOperationException has this specification and implemention

    /**
     * Constructs an UnsupportedOperationException with no detail message.
     */
    public UnsupportedOperationException() {
    }



The parent class - RuntimeException is like this

    /** Constructs a new runtime exception with {@code null} as its
     * detail message. The cause is not initialized, and may subsequently be
     * initialized by a call to {@link #initCause}.
     */
    public RuntimeException() {
        super();
    }

So RuntimeException is clear that the detail message is null.
And the parents -> Exception -> Throwable are the same.

But UnsupportedOperationException is less clear - "no detail message" could mean null, or it could be interpreted as an empty string.
However it does behave as per its super-classes.
public class USOE {

  public static void main(String[] args) {
    UnsupportedOperationException e = new UnsupportedOperationException();
    System.out.println(e.getMessage());
  }
}

% java USOE.java
null

That vagueness could in part explain why a subclass of UnsupportedOperationException - java.awt.HeadlessException says
"Constructs new HeadlessException with empty message" - so that in some cases it will also report null, not "".