distinguish trait absence from falsy trait values

Author: khvn26

flag_engine/segments/evaluator.py

@@ -379,7 +379,7 @@ def _get_context_value_getter(
         return partial(_get_trait_value, trait_key=property)
 
     def getter(context: EvaluationContext) -> ContextValue:
-        if trait_value := _get_trait_value(context, property):
+        if trait_value := _get_trait_value(context, property) is not None:
             return trait_value
         if typing.TYPE_CHECKING:  # pragma: no cover
             # Ugly hack to satisfy mypy :(