[std] fix some `return null` cases

std/cpp/NativeXml.hx

@@ -356,14 +356,14 @@ class Xml {
       if (_attributes==null)
          _attributes = {};
       Reflect.setField (_attributes, att, value );
-      return null;
+      return;
    }
 
    public function remove( att : String ) : Void{
       if( nodeType != Xml.Element )
          throw "bad nodeType";
       Reflect.deleteField( _attributes, att );
-      return null;
+      return;
    }
 
    public function exists( att : String ) : Bool {
@@ -422,7 +422,7 @@ class Xml {
       if( x._parent != null ) x._parent._children.remove(x);
       x._parent = this;
       _children.push( x );
-      return null;
+      return;
    }
 
    public function removeChild( x : Xml ) : Bool {
@@ -439,7 +439,7 @@ class Xml {
       if( x._parent != null ) x._parent._children.remove(x);
       x._parent = this;
       _children.insert( pos, x );
-      return null;
+      return;
    }
 
    public function toString() : String {