Prevent bug in upgrade diff for empty new version

apparently this is quite the rare case..but can happen

tailbone/views/upgrades.py

@@ -400,6 +400,10 @@ class UpgradeView(MasterView):
         return projects
 
     def get_changelog_url(self, project, old_version, new_version):
+        # cannot generate URL if new version is unknown
+        if not new_version:
+            return
+
         projects = self.get_changelog_projects()
 
         project_name = project