Allow 'None' as a default_value for _yaml.node_get

buildstream/_options/option.py

@@ -62,7 +62,7 @@ class Option():
     #                 the option
     def load(self, node):
         self.description = _yaml.node_get(node, str, 'description')
-        self.variable = _yaml.node_get(node, str, 'variable', default_value='') or None
+        self.variable = _yaml.node_get(node, str, 'variable', default_value=None)
 
         # Assert valid symbol name for variable name
         if self.variable: