Make Objects.requireNonNull() throw IllegalArgumentException

and not NullPointerException. Altough this differs from
java.util.Objects behavior, throwing an IllegalArgumentException
appears more sensible and makes it easier to catch it in Smack's
parsing function.

smack-experimental/src/test/java/org/jivesoftware/smackx/reference/ReferenceTest.java

@@ -98,7 +98,7 @@ public class ReferenceTest extends SmackTestSuite {
 
     @Test
     public void typeArgumentNullTest() throws URISyntaxException {
-        assertThrows(NullPointerException.class, () ->
+        assertThrows(IllegalArgumentException.class, () ->
         new ReferenceElement(1, 2, null, null, new URI("xmpp:test@test.test")));
     }