and fix the array-based join method as well

src/main/org/apache/tools/ant/util/StringUtils.java

@@ -297,6 +297,9 @@ public final class StringUtils {
      * @return the joined string
      */
     public static String join(Object[] array, CharSequence separator) {
+        if (array == null) {
+            return "";
+        }
     	return join(Arrays.asList(array), separator);
     }
 

src/tests/junit/org/apache/tools/ant/util/StringUtilsTest.java

@@ -182,10 +182,15 @@ public class StringUtilsTest {
     }
     
     @Test
-    public void testJoinNullArray() {
+    public void testJoinNullCollection() {
     	assertEquals("", StringUtils.join((Collection<String>)null, ", "));
     }
 
+    @Test
+    public void testJoinNullArray() {
+    	assertEquals("", StringUtils.join((String[])null, ", "));
+    }
+
     @Test
     public void testJoinNullSeparator() {
     	assertEquals("abc", StringUtils.join(Arrays.asList("a", "b", "c"), null));