fixed test case + strict checking for atom

Author: Burak Emir

src/library/scala/xml/Atom.scala

@@ -21,6 +21,10 @@ import compat.StringBuilder;
 [serializable]
 class Atom[+A]( val data: A ) extends SpecialNode {
 
+  data match {
+    case null => new IllegalArgumentException("cannot construct Atom(null)")
+    case _ =>
+  }
   final override def typeTag$:Int = -1;
 
   /** the constant "#PCDATA"

test/files/jvm/xml03syntax.check

@@ -1,6 +1,5 @@
-<hello>()</hello>
 passed ok
-<hello>()</hello>
+passed ok
 passed ok
 <hello>world</hello>
 passed ok

test/files/jvm/xml03syntax.scala

@@ -13,13 +13,18 @@ object Test {
     import NodeSeq.view
     import testing.UnitTest._
 
-    val x0 = <hello>{}</hello>
-    val x00 = <hello>{ }</hello>
+    val xNull = <hello>{null}</hello> // these used to be Atom(unit), changed to empty children
+
+    assertSameElements( xNull.child, Nil )
+
+    val x0 = <hello>{}</hello> // these used to be Atom(unit), changed to empty children
+    val x00 = <hello>{ }</hello> //  dto.
+
     val xa = <hello>{ "world" }</hello>
 
 
-    assertEquals( handle[Unit](x0),  {} )
-    assertEquals( handle[Unit](x00), {} )
+    assertSameElements( x0.child,  Nil )
+    assertSameElements( x00.child, Nil )
     assertEquals( handle[String](xa),"world" )
 
     val xb = <hello>{ 1.5 }</hello>