fix: subresource can be empty

Author: necusjz

src/azure-cli-core/azure/cli/core/aaz/_selector.py

@@ -55,6 +55,4 @@ def set(self, value):
 
     def required(self):
         value = self.get()
-        if not has_value(value):
-            raise UserFault("ResourceNotFoundError")
         return value