Support symbol None (from old versions)

Fix #732

gns3server/controller/node.py

@@ -204,6 +204,9 @@ class Node:
 
     @symbol.setter
     def symbol(self, val):
+        if val is None:
+            val = ":/symbols/computer.svg"
+
         # No abs path, fix them (bug of 1.X)
         if not val.startswith(":") and os.path.abspath(val):
             val = os.path.basename(val)