OV-7. When converting ASN object to string, handle None case.

openvisualizer/openType/typeAsn.py

@@ -22,6 +22,8 @@ def __init__(self):
         self.asn = None
 
     def __str__(self):
+        if self.asn is None:
+            return '?'
         return '0x{0}'.format(''.join(["%.2x"%b for b in self.asn]))
 
     def __eq__(self, other):