Calling Java varargs method with single null argument?

A Test Case to illustrate this:

The Java code with a vararg-taking method declaration (which happens to be static):

public class JavaReceiver {
    public static String receive(String... x) {
        String res = ((x == null) ? "null" : ("an array of size " + x.length));
        return "received 'x' is " + res;
    }
}

This Java code (a JUnit4 test case) calls the above (we are using the test case not to test anything, just to generate some output):

import org.junit.Test;

public class JavaSender {

    @Test
    public void sendNothing() {
        System.out.println("sendNothing(): " + JavaReceiver.receive());
    }

    @Test
    public void sendNullWithNoCast() {
        System.out.println("sendNullWithNoCast(): " + JavaReceiver.receive(null));
    }

    @Test
    public void sendNullWithCastToString() {
        System.out.println("sendNullWithCastToString(): " + JavaReceiver.receive((String)null));
    }

    @Test
    public void sendNullWithCastToArray() {
        System.out.println("sendNullWithCastToArray(): " + JavaReceiver.receive((String[])null));
    }

    @Test
    public void sendOneValue() {
        System.out.println("sendOneValue(): " + JavaReceiver.receive("a"));
    }

    @Test
    public void sendThreeValues() {
        System.out.println("sendThreeValues(): " + JavaReceiver.receive("a", "b", "c"));
    }

    @Test
    public void sendArray() {
        System.out.println("sendArray(): " + JavaReceiver.receive(new String[]{"a", "b", "c"}));
    }
}

Running this as a JUnit test yields:

sendNothing(): received 'x' is an array of size 0
sendNullWithNoCast(): received 'x' is null
sendNullWithCastToString(): received 'x' is an array of size 1
sendNullWithCastToArray(): received 'x' is null
sendOneValue(): received 'x' is an array of size 1
sendThreeValues(): received 'x' is an array of size 3
sendArray(): received 'x' is an array of size 3

To make t