Make bool_field property return False if no value is found in the dict.

debexpo/plugins/api.py

@@ -96,7 +96,7 @@ def test_result(cls):
 def bool_field(name):
     """ Property exposing a string ('true' or 'false') as a bool. """
     def fget(instance):
-        return True if instance[name] == 'true' else False
+        return True if instance.get(name, 'false') == 'true' else False
 
     def fset(instance, value):
         if not isinstance(value, bool):