Add another assertion documention the current behavior of StringUtils.join

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/lang/trunk@1647902 13f79535-47bb-0310-9956-ffa450edef68

src/test/java/org/apache/commons/lang3/StringUtilsTest.java

@@ -425,6 +425,8 @@ public class StringUtilsTest {
         assertEquals("", StringUtils.join(Arrays.asList(EMPTY_ARRAY_LIST).iterator(), SEPARATOR));
         
         assertEquals(TEXT_LIST, StringUtils.join(Arrays.asList(ARRAY_LIST).iterator(), SEPARATOR));
+
+        assertNull(StringUtils.join(Arrays.asList(NULL_TO_STRING_LIST).iterator(), SEPARATOR));
     }
 
     @Test