Fix crash on empty xfail parameter set with --runxfail

changelog/4497.bugfix.rst

@@ -0,0 +1 @@
+Fixed crash when running an empty ``xfail``-marked parameter set with ``--runxfail``; it now fails with a clear message.

src/_pytest/python.py

@@ -1228,7 +1228,12 @@ def id(self) -> str:
 
 
 def get_direct_param_fixture_func(request: FixtureRequest) -> Any:
-    return request.param
+    try:
+        return request.param
+    except AttributeError:
+        # Empty parameter set under --runxfail has no value to run with:
+        # fail cleanly instead of crashing (#4497).
+        fail("no direct parameter (empty parameter set)")
 
 
 class DirectParamFixtureDef(FixtureDef[FixtureValue]):

testing/test_skipping.py

@@ -545,6 +545,25 @@ def test_this():
         """
         )
 
+    def test_runxfail_with_empty_parameter_set_mark_xfail(
+        self, pytester: Pytester
+    ) -> None:
+        """--runxfail on an empty xfail set fails cleanly, not crashes (#4497)."""
+        p = pytester.makepyfile(
+            """
+            import pytest
+
+            @pytest.mark.parametrize(("a", "b"), ())
+            def test(a, b):
+                pass
+            """
+        )
+        result = pytester.runpytest(
+            p, "--runxfail", "-o", "empty_parameter_set_mark=xfail"
+        )
+        result.stdout.fnmatch_lines(["*no direct parameter (empty parameter set)*"])
+        result.assert_outcomes(errors=1)
+
     def xtest_dynamic_xfail_set_during_setup(self, pytester: Pytester) -> None:
         p = pytester.makepyfile(
             """