do not fail validation on empty object_id

Signed-off-by: Nico Schottelius <nico@brief.schottelius.org>

lib/cdist/core/cdist_object.py

@@ -166,6 +166,8 @@ class CdistObject(object):
         Remove leading and trailing slash (one only)
         """
 
+        # Allow empty object id for singletons
+        if self.object_id:
             # Remove leading slash
             if self.object_id[0] == '/':
                 self.object_id = self.object_id[1:]