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)

gns3server/schemas/node.py

@@ -146,7 +146,7 @@ NODE_OBJECT_SCHEMA = {
         "label": LABEL_OBJECT_SCHEMA,
         "symbol": {
             "description": "Symbol of the node",
-            "type": "string",
+            "type": ["string", "null"],
             "minLength": 1
         },
         "width": {