fix Asset.pm so that it returns undef if no valid Asset className is passed

docs/changelog/7.x.x.txt

@@ -5,6 +5,8 @@
  - Fixed the test skeleton
  - Fixed some bugs regarding Search relevance sorting (Len Kranendonk / www.ilance.nl)
  - Added an option to override the session cookie name.
+ - Fix Asset.pm so that if no className is passed primeval Assets are not
+   returned.
 
 
 7.0.4

lib/WebGUI/Asset.pm

@@ -1406,6 +1406,7 @@ sub new {
 		}
 		$class = $className;
 	}
+	return undef if $class eq 'WebGUI::Asset';
 	my $cache = WebGUI::Cache->new($session, ["asset",$assetId,$revisionDate]);
 	my $properties = $cache->get;
 	if (exists $properties->{assetId}) {