Fill in missing empty attr lists

op attribute lists are referenced without checking if they are present.
Attibute list can be a optional property so this patch add them initially
if missing.

Signed-off-by: Zibo Gong <gongzib@outlook.com>

Author: zb-gong

ynl-gen-cpp.py

@@ -924,6 +924,14 @@ def new_attr(self, elem, value):
 
 class Operation(SpecOperation):
     def __init__(self, family, yaml, req_value, rsp_value):
+        # Fill in missing operation properties (for fixed hdr-only msgs)
+        for mode in ["do", "dump", "event"]:
+            for direction in ["request", "reply"]:
+                try:
+                    yaml[mode][direction].setdefault("attributes", [])
+                except KeyError:
+                    pass
+
         super().__init__(family, yaml, req_value, rsp_value)
 
         self.render_name = c_lower(family.name + "_" + self.name)