Fix an additional case for python-poetry/poetry#4402

Resolves python-poetry/poetry#4402

Author: morne

poetry/core/version/markers.py

@@ -297,6 +297,9 @@ def validate(self, environment: Dict[str, Any]) -> bool:
         if self._name not in environment:
             return True
 
+        if self._constraint.is_empty():
+            return True
+
         return self._constraint.allows(self._parser(environment[self._name]))
 
     def without_extras(self) -> MarkerTypes: