Minor bug fix to deal with virt_nodes entries with null tarfiles/rpms.

tbsetup/tarfiles_setup.in

@@ -2,7 +2,7 @@
 
 #
 # EMULAB-COPYRIGHT
-# Copyright (c) 2003, 2004, 2006, 2007 University of Utah and the Flux Group.
+# Copyright (c) 2003-2008 University of Utah and the Flux Group.
 # All rights reserved.
 #
 
@@ -140,6 +140,10 @@ while (my ($vname, $rpms, $tarfiles) = $result->fetchrow()) {
     if (! defined($physnode)) {
 	$physnode = $experiment->VnameToPmap($vname);
     }
+    $rpms = ""
+	if (!defined($rpms));
+    $tarfiles = ""
+	if (!defined($tarfiles));
 
     #
     # Go through the list of RPMs looking for files to fetch