Allow "extra" to be None in the marker environment

This is for compatibility with older behaviour of this API.

src/packaging/markers.py

@@ -237,5 +237,9 @@ def evaluate(self, environment: Optional[Dict[str, str]] = None) -> bool:
         current_environment["extra"] = ""
         if environment is not None:
             current_environment.update(environment)
+            # The API used to allow setting extra to None. We need to handle this
+            # case for backwards compatibility.
+            if current_environment["extra"] is None:
+                current_environment["extra"] = ""
 
         return _evaluate_markers(self._markers, current_environment)

tests/test_markers.py

@@ -7,6 +7,7 @@
 import os
 import platform
 import sys
+from typing import cast
 
 import pytest
 
@@ -256,6 +257,20 @@ def test_compare_markers_to_other_objects(self):
     def test_environment_assumes_empty_extra(self):
         assert Marker('extra == "im_valid"').evaluate() is False
 
+    def test_environment_with_extra_none(self):
+        # GIVEN
+        marker_str = 'extra == "im_valid"'
+
+        # Pretend that this is dict[str, str], even though it's not. This is a
+        # test for being bug-for-bug compatible with the older implementation.
+        environment = cast("dict[str, str]", {"extra": None})
+
+        # WHEN
+        marker = Marker(marker_str)
+
+        # THEN
+        assert marker.evaluate(environment) is False
+
     @pytest.mark.parametrize(
         ("marker_string", "environment", "expected"),
         [